Par 4s are normally the 'teeth' of most courses. Par 5s tend to be wide open and you got 3 swipes to get on, and par 3s, well, unless they are long or beset with hazards, you should at least be around the green off the tee. Par 4s, you never know. Quite often, they test the largest assortment of shots in your arsenal. Par 4s.are where the architect takes his revenge.
I just randomly grabbed these two to quote even though a lot of others say close to the same thing. I think they are all right. You have no trouble on par 3's because you have a perfect lie and (usually) a mid iron or less in your hand. Pretty tough to get too far off track with that. Par 5's, assuming you keep that tee shot in bounds and out of the water, it doesn't really matter how far you hit it ... you just have two easy shots to get to the green. Par 4's on the other hand, you have no room to miss. I bet, though, that the reason why you do poorer on the 4's is mental.
Here's an example:
Say you have a 490 yard par 5 and you hit a really nice 3 wood in the middle of the fairway that goes approximately 240. Now you have a perfect lie are are 250 out. Too far to reach, so what do you do? (I think) You pull out a 7 or 8 iron and try to stick yourself in the middle of the fairway up closer to the green ... say 100 yards out, right? Then all you have to do is get down in 4 with a pitching wedge from 100 to get your 6. You can do that with your eyes closed.
OK, now say that you're on a 420 yard par 4. You, again, pull your 3 wood and this time you get a little under it and sky it, say 180 but straight. Now you are in almost the exact same situation as the last hole, and here's where I'm guessing the problem lies; you know its a par 4 and you let that cloud your thinking as to how to play the next shot. You are "supposed" to be at the green in two shots, so you are more inclined to try and hit a 3 wood again, or something else long, and that is where you can put yourself into more trouble. Just because the scorecard says something different doesn't mean you should play the hole differently. If you call it a par 5 from that point forward, then lay up with your 7 iron ... there is no reason why you couldn't coast to a 6, and perhaps a 5 or even a 4.
I tend to almost agree with PirateJim here. I don't know that I'd recommend turning the hole into a par 5 in your head at the tee ... but I definitely agree that after a less than perfect tee shot you should do that.
*(You should also examine your course a little too, because that might be skewing your results. Just off the top of my head ... out of 4 par 5's, only 2 of them have OB potential, correct? Whereas, out of 10 par 4's I believe that there is OB on all 10, correct? You might want to take a closer look at the penalty strokes hole by hole too, because it could be as simple as that)