Wow, that's a pretty badass compression loss. Here is what I would do if this were my car. Do a compression test on ALL the other cylinders, even the rear bank, to get a good feel on how every thing is shaping up. Any cylinder that reads more than a 15psi difference with the others gets a leakdown test (including #4, which if I remember correctly is the one giving you issues). If everything else looks alright and the leakdown test on #4 points to a valve problem, pull the front head and check it out. Once it's off a valve problem will most likely be apparent. Now, if this were my car, I'd rework the front head and call it a day. That is assuming, of course, that all my other testing showed that the rest of the engine is fairly sound (i.e. good compression, little oil consumption, and no smoke). The reason I say this is that in reworking the front head there is nothing else in your way. If you were dealing with the rear bank, you might as well do both because the rear is such a pain to get to without tearing everything apart. But on the other hand, if you like your car and want to keep it a while, you may be in the position to go all in an get the whole shebang, like checking out the crank & rod bearings, replacing rings, and the like. But if this were my ride I'd probably just address the problem in the #4 cylinder in the front bank. It's practically begging to get yanked off anyway. Hope this helps.