you risk serious engine damage if you run only 3 bolts. the bike will probably run fine until it gets hot.. then air will leak in and out of the head gasket area and the cylinder base area. This fresh air will lean the air:fuel mixture and overheat something..
First thing to happen will probably be it gets the head so hot it warps upward on the corner where there is no bolt.. But a warped head is not the worst that can and will happen if you insist on continuing to ride it.
If the bolt's engine threads got stripped, bring the engine to a mechanic who can install a HeliCoil or similar for a few dollars..
If the bolt snapped off, remove what's left of it in the hole and get a new bolt. (You can remove it in a few ways.. ask if interested)
And buy a torque wrench.. and follow the torque specifications in the manual.