RE: 4 stroke, for speed planes?
Much depends on your definition of speed... for some people "speed" is faster than all the regular stuff at the field, say perhaps 100-120mph. Get used to flying fast stuff regularly and 110mph seems pretty normal.
2 stroke or 4 stroke aside, it starts to take effort to achieve numbers much above 150, although that is still an off-the-shelf number reachable by many for a reasonable budget. In terms of prop pitch and rpm, 150mph needs 12" pitch at >13,000 rpm. Now think of that compared to available props and typical four stroke engines.. hmm, what to do. Whereas 120mph at 12" pitch translates to a more reasonable figure of a bit over 10,500rpm. Theoretical numbers of course, but then again most props have aerodynamic pitch higher than the listed number anyway, so it tends to come out in the wash.
There's little stopping the production of a higher rpm model four stroke, but of course that is not where the market = money is.