Hi John.  In my opinion it really depends upon your setup, the clubs in your bag, and most importantly the distance gaps between your clubs.  I believe the most important thing in your setup is that you can cover the various distances evenly throughout your set and you don't have any distance "holes".  Look at he distance range with your 5 iron and your 4 iron and ask yourself, "Can I hit the distances I am currently hitting with my 4 iron with the hybrid I am considering replacing it with?"

When looking at my clubs, I have several different "options" that I can switch in and out of my bag.  I keep AP2 irons through a 3-iron that I can use as well as 3 hybrids at different lofts.  I tend to switch these clubs in and out of my bag based on how I am hitting the ball, the course, etc.  I tend to stick with a setup for a while and, if I feel a tweak is needed, shift the "magic 14" that I keep in my bag around a bit.

On the top end, I currently keep AP2 irons through a 4-iron in my bag, a 20 degree hybrid and a 15 degree 3-wood.  This covers everything I need at the top end of my range.  Go out to the range and try the different permutations you are considering and see what types of distances you are getting with the various setups.  At the end of the day, it its about hitting the ball a certain distance in a consistent manner.  I would also get with your local Titleist fitter to make sure the clubs are set up properly for your swing.

I carry a 7W and a 4H (21F/24H).  There is a difference in distance.  I have a 5I and a 5H.  The difference is I can get more consistent height with the 5H so I can control the distance better.  Would that make a difference if your 4I went 180-200 and the 4H went 185-190 instead?  If you are looking to carry another wedge, you can look at a 21 H and choke down for what your 4I does today.  Titleist makes life so hard by so many choices. 

At my home course, I have a couple of par 3's where a 21* hybrid is the perfect club for the yardage for me.  Unfortunately, my next club is a 5 iron, resulting in a 20 something yardage gap.  I haven't quite mastered this choking down on a hybrid concept yet, so when that yardage comes up (fortunately not that often), I find myself in a pickle.  So I'm with the other fellows - make sure the yardage gaps work for you.

As for the comparison, I think the 7 wood is  probably closer to a 21* hybrid (~195 yards), at least for my swing.  As always, YMMV.  But I prefer the hybrid, as I've found it a little more versatile than the 7 wood, although there is nothing like the soft landing of a shot from 200 yards out with a 7 wood.

I agree a hybrid is a great alternative to a fairway metal for consistency and a higher trajectory for shots into a green.

Unless your a scratch player I would switch , I found I gained about 12 yards from a good lie and a higher and softer flight.Also they are ,IMO , much easier to get airborne from the rough

Your call John.  I had a 909 Hybrid and ended up getting my AP2's with a 4 iron in the bag and I love it.  You're the one than has to play it.  Hybrids, in my opinion hybrids get the ball up better, but you can't work them as much.  If you struggle with your 4 iron, get a hybrid.  If you like your 4 iron, keep it!