Not a 4 stroke, but I had a KMX 125 which was great and with a couple of mods pulled better han th e old 86 CR 125 I had at the time - downside would be it's physically quite a small bike.
The other one I'd look at isthe old KTM LC2 I think it was - this had the DT125R engine in it (with the electronic powervalve I think) and was a bit more trail orintated than the DT (but again a two stroke)
I'd love another bike and am seriously considering getting a 125 again (I'm 28, CBT ran out years ago and stupidly never got round to taking a proper test so have been bikeless for a long while).
I did have atest ride on an older (late 90's ) XL 125 R - nice bike but very leisurely compared to a two banger.
Trying to talk SWMBO around but getting nowhere.