I am looking to upgrade my radio too, and from reading I think I too want to go Futaba; from reading here it appears that the futaba fast system is better than the spektrum. So I am tossing up on 3PM, 3PKS and 4PKS.
I am hoping that with the release of te 4PK, some hobby shops might put a clearance on the 3PKS and I can get a good deal.
As far as differences and features, I know that the 4PK has the module built in unlike the 3PKS. This link has some good discussions on it:

http://www.rctech.net/forum/showthread.php?p=3771272