1999 5.7 mercruiser (Fuel injected) backfiring issue

[dippasan]

I performed a compression test on the engine this past Saturday.
7 cylinders showed 175 psi but #4 cylinder has 45 psi.

In talking to my mechanic, in the 90's for some reason, it was a common problem to "tulip" a valve. Probably an intake valve hence the backfiring through the throttle body. I removed the valve cover and ran the motor and saw nothing out of the ordinary with the spring, push rod or rocker (as you might expect because the problem is on the combustion chamber side).

So it looks like I have a valve job in my immediate future. I'm just a little stumped. I've worked on cars my whole life and I've never had a valve fail on a motor. Especially a Chevy 350 which are typically bullet proof. These motors have 360 hours and run as sweetly as can be with no sign or indication that a valve would fail. Strange!