I tried to overclock my cpu from3.8ghz to 4ghz and now it won't start
Yep. That's what happens when you don't know what you're doing.
You'll need to somehow reset your CMOS configuration. How you do it depends on your mobo, which you didn't tell us what it is. On my Gigabyte Mobo, you short two jumpers and that resets, but it may be different on yours.
well then. that's a lesson learned lol. find the reset jumper on your motherboard it should reset the OC settings.
'If it ain't broke, don't fix it'.