Do you live near a track?
Then keep the 335 for a while longer and get a used E36 M3 (several always for sale in the Roundel classifieds for less than $20k). Enjoy it at the track and learn to milk it for all it's worth. Or if you're already a trained track rat, spend $25k on one of the BMW club racers that are always for sale at the BMW racing club page.
Then when the M4 is out, you will be in a good position to try out used E92s and the new M4, and pick whichever you like best. I'm assuming BMW will fix the M4 so the engine doesn't overheat and limp out at the track like the N54-55s do now. If you like the E92 better you will get it used and save some money. You can always re-sell the track car for 95% of what you paid for it.