Do you need to hit and hold greens with a 4 iron more than fill a gap in the distance between the 4I and 5W? The obvious answer is a 3H to fill the distance gap, but if your 4I is only 5 yards more than your 5I, you may want a 4H.

Since you already have a 4-iron and a 3-wood, you need to fill the yardage gap between those two clubs.  For instance, if your 4-iron distance is 200 and your 3-wood is 230 then you'd want something to cover the 215-220 yardage.  It really doesn't matter which one it is as long as it fills the gap that you need filled, kind-of like a wedge.  You could also add a 5-wood if the hybrid doesn't net the results you're looking for.

Point is, cover the yardage that's good for you and get the club that's easiest for you do execute it with...