Nonetheless, a short side-by-side comparison of the findings of the two reports can be instructive for understanding what the IPCC really says. At the core of the third assessment report's predictions on the range of future surface warming are six scenarios, taken from the IPCC's special report on emissions scenarios. These scenarios are crucial for understanding what the report actually states, so with that in mind here is a brief out-take from that report:

By 2100 the world will have changed in ways that are difficult to imagine - as difficult as it would have been at the end of the 19th century to imagine the changes of the 100 years since. Each storyline assumes a distinctly different direction for future developments, such that the four storylines differ in increasingly irreversible ways. Together they describe divergent futures that encompass a significant portion of the underlying uncertainties in the main driving forces. They cover a wide range of key "future" characteristics such as demographic change, economic development, and technological change. For this reason, their plausibility or feasibility should not be considered solely on the basis of an extrapolation of current economic, technological, and social trends.

• The A1 storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building, and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B).
• The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results in continuously increasing global population. Economic development is primarily regionally oriented and per capita economic growth and technological change are more fragmented and slower than in other storylines.
• The B1 storyline and scenario family describes a convergent world with the same global population that peaks in midcentury and declines thereafter, as in the A1 storyline, but with rapid changes in economic structures toward a service and information economy, with reductions in material intensity, and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives.
• The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to economic, social, and environmental sustainability. It is a world with continuously increasing global population at a rate lower than A2, intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented toward environmental protection and social equity, it focuses on local and regional levels.

The estimation in the third assessment report (TAR) contains a range of estimates of the average level of wordlwide surface temperature change in the year 2100 on these scenarios that runs from 1.4 to 5.8 degrees celsius, depending on different models. The range of estimates in the fourth assessment report (AR4) runs from 1.1 to 6.4 degrees celsius in the 2090-2099 period, including estimates of 'likely' scenarios. Such probabilistic estimates were not available in the third assessment report, so these numbers represent different methodologies.

I'll hazard the guess that the "best estimates" for the scenarios in AR4 can be compared to the harmonised estimates in TAR. The numbers:

This represents something of a minor downward adjustment. On the other hand, the generalised range of "likely" temperature changes according to the summary for policy makers is 2 to 4.5 degrees, which is the exact same range as that for the harmonised estimates for the scenarios in the TAR.

Other issues.

It has been noted here that the IPCC uses optimistic estimates of the world's fossil fuel resources and therefore would project temperature changes that are unrealistic. However, I think that these estimates will only really be relevant for the A1FI scenario.

There is a notable lag between the emission of carbon dioxide and the effect it has on surface temperatures and carbon dioxide is not the only relevant greenhouse gas (it is expected to account for about 55% of total current warming, the other gases account for about 30%). The IPCC report also has a "best estimate" of the temperature change for a scenario where the athmospheric concentrations of all greenhouse gases remain equal to 2000 levels, and that is 0.6 degrees celsius. This reflects the lag between the level of atmospheric concentration and the associated warming, and also the degree to which the process of global warming has become self-driving.

The degree to which climate change is self-driving increases with the expected temperature increase, for the A2 scenario the IPCC estimates that feedbacks in the carbon cycle alone will cause more than 1 degree of  the warming. In order to stabilise at a given level of warming in 200 to 300 years, disproportionate cuts in emissions are necessary and this imbalance will increase the longer we postpone these cuts.

Another controversy has been raised over the expected degree of sea level rise (see this post by Roger Pielke Jr.). Near as I can tell, the estimate has decreased from a maximum of 0.88 meters rise to 0.79 meters assuming a linear increase of the degree of melting with the degree of temperature change. It seems that the TARs numbers used that method. So-called "dynamic effects" like "changes due to changes in ice streams, calving, grounding line movement" are not included in either of these numbers, but these effects are now thought to be potentially significant. At the time of the third assessment report, they were largely judged to be negligable. Dynamic effects are not included because the science on the topic is not yet fully established.

Because of changed methodologies, a more precise characterisation of the fourth assessment report is only possible once the first part of the report has been released. A complete discussion of the desired policy responses can be had once we have the second and third parts of the report.

AR4 Summary for Policymakers (.pdf)

IPCC Reports page