I think that 4.00GHz is reasonable with a good air cooler. I have an i5 3.33GHz @ 4.00GHz that gets up to about 80°C during a gaming session. Just monitor your temperatures in SpeedFan or RealTempGT to make sure you aren't sustaining temperatures above 90° for an extended period of time in the course of the most demanding thing you use your PC for.
PLEASE don't go to 90c or even 80c........WAY too hot.
If you run a high end air (NH-D14, Megahalems w/dual fans, etc.) you can run at 4.0 Ghz as long as your chip will do this with decent voltages. Stay with the Intel thermal spec, low 70's as your max and you'll be fine.