These are indeed gloomy days for the Republican Party. Just two months after a disastrous election, ten percent of their Senate contingency is calling it quits for 2010 (twenty percent of those up for reelection). Those retiring are Kit Bond (MO), Sam Brownback (KS), Mel Martinez (FL), and George Voinovich (OH). So far the only Democratic "retirement" is that of seat warmer Ted Kauffman who plans to make way for Biden fils.
What is perhaps more troubling for the party is the difficulty the party will have in retaining all but the Kansas seat. Obama won Florida and Ohio and barely lost in Missouri. But the longer term damage may be even greater. All of the retirements are members from the most moderate half of the Republican party (Brownback is indeed socially conservative, but significantly more moderate on other issues). Thus, these retirements will further erode moderate wing of the Republican party and further polarize the Senate.
Update: Chris Bowers makes the point that the four retiring senators may be more likely to support President Obama's economic iniatives than the non-retiring senators. He bases this prediction in part on behavior of retirees on the TARP and Auto Bailout votes. I also found evidence for such an effect here and here.