I really want this but have yet to play mgs4, owned it for years but the day of my life devoted to cut scenes has always put me off.

So how important is it to finish mgs4 ?

As someone who has not played Phantom Pain it seems like it isn't important for you to have played MGS4, but instead it is much more important to have played MGS3, Peace Walker and Ground Zeroes before playing Phantom Pain.

Five hours into PP and I haven't seen a cutscene since the intro...I have a feeling I'll want to play 3 again after V. When V has come 2 4 real.

Ideally you'd want to play MGS4, if only because it marks the end of the Patriots, whereas MGSV seems like (not spoiler, just speculation on my part) it will feature their genesis.
However, the actual plot is focussed way more on Big Boss' story arc, and if you have played MGS3, and know what happens in PW and MG1+2 you should be okay. Bear in mind I have not finished the game.

In order to get the most out of the second act you should probably play all of them

I just watched Drew play them, then watched a YouTube recap. It's fine. I haven't played a Metal Gear game since the 80s.

depends what your asking, should i play mgs4 before 5 to get caught up with the story? then not really because it's chronologically the last game in the series, then you should be ok to play 5 before 4 if that's what you really wanted to do. If your asking something else, then I would play it, it's a fun game, even with the long cutscenes.

From what I have seen, Ground Zero and Peace Walker are a lot more relevant to the story of 5 than the other games in the franchise.

Well, i'm sure stuff in phantom pain will spoil stuff in 4 since it's a prequel but if your not that concerned with ever playing 4 it probably won't matter. But you should play 4, it's great and ties up solid snakes story completely. Playing 3 and peacewalker(or wathing the movie of peacewalker) are pretty crucial to know wtf is going on though. Also Ground Zeroes is pretty much a small part of the game that I think should be played before hand, maybe not for full price though.

You can understand Phantom Pain easily without having played 4 but Phantom Pain will lessen the impact of certain parts of 4 because it spoils or refers to events that were introduced in 4.

4 isn't that long. I say play it. It's an incredible game.

like any game it's fairly unimportant.