You should slowly go up in speed so 4.1GHz then 4.2GHz and leave it for a hour or so to check stability or if you have AMD(not sure if works on other CPUs) you can use auto clock with "Amd Overdrive" Let it run and it'll find a good stable Clock. I belive Overclock is worth everybit, Why pay more if you have more if you can overclock? Last time I tried, it boosted my game from 10-20fps to 30-50 pretty steady

BTW if it's Restarting it's probably the computer stopping your computer from overheating and damaging itself