As for the Four that will be retired I'm sure that Gulfstream put in a request for the Aircraft to be part of the bill for the new jets. They will bring them back to the factory and depending on how many Flight Hours are on the Airframe either rebuild them and send them back tot eh USAF or keep them outright for test airframes. I do know that two of the very first 37A-I have allot of Time on the Airframe and are getting close to critical time on the Wing and Main Airframe.
The new aircraft will be the C-37B which is the Military version of the Gulfstream G550 which has better engine and range then the 37As. Plus all of the 37B have New Updated Glass Cockpits and weather radar.
As for are they improvement yes the have a better fuel range burn less Fuel then the 37A and are little bit larger and have update cockpit systems.
As for your last question I think in these times of Hyprerpartisin Politics it is nice to know that sometime Congress does something that is the right thing to do. Will it deflect debate no but in the end it is the correct thing to do replace an older less efficent aircraft that has served the Country well with new and updated airframes that will serve the Country for a good 10 Year or more.
One item folks seem to forget is that these aircraft from the 87th Special transport Wing put in allot of Hours every year which adds up quickly on an airframe time in service.