I'll 2nd a valve problem. Either the intake or exhaust valve isn't closing all the way. The valve could be bent and sticking, burnt and not sealing or has some carbon or other material keeping it from closing or adjusted too tight.
Remove the valve cover and go through the valve adjusting procedure on cyl #4.
If that doesn't correct it then do a leak down test on cyl #4 and listen to the exhaust and intake to determine where the air is leaking out.
Good luck
Dave
