Ford F150 - 89 4.9 valve trouble question

I have an 89 4.9 with 170,000 that began running rough, lost power very suddenly. I did a compression check and found cylinder 4 with less than 25 psi and all other cylinders at >150 psi. I pulled the valve cover and found #4 exhaust pushrod free from the rocker. I replaced the bad rod, put her back together and it ran fine and seemed fixed. I drove it about a mile and my problem returned, no compression in 4 again. I took it apart once again and expected to find the pushrod dropped but it was fine. Am I looking at a bent valve, bad cam, lifter, or none of the above????? I'm pretty new at this and still learning. I need input please!!!
Thanks,
indycircuits

Pull the SIDE cover (#13) & see what's going on.