Cessna upgraded CJ2 in to CJ2+ the main difference being FADEC's on engines, some more power, a little bit betterperformance, but they throw away thrust attenuators ( one of the reasons that NetJets doesn't wan't to put them in to the fleet ), some better avionics ( Collins 3000 ), plus mome minor differencies...
All this CJ3 had from the begining so there is no need to upgrade it ot some + variant. If I have to retard throtles at F410 not to exced the speed I don't know what more I need, realy spacious cabin....
CJ4 is something totaly new, swept wing, even stronger engines...but I belive the price will bring it in to the class of bigger aircrafts.
BTW, does any one know if CJ4 will be single pilot ACFT..it is shure nice to cruise single with Cj1/2/3's, no one to bother's you there on the right seat
cheers