We have no way of knowing what your specific "gap" distances are. If there is 10-15 yards between your two fairway metals and another 10 yards between that 21 degree and your 4 iron, I would say you did not have a gap and are good to go. If you have 20 yards between your 21 degree fairway and your 4 iron, maybe then you hit a couple of different test hybrids to see which one gives you that correct distance between those two clubs. You can't really trust just building your set based on the listed lofts. I currently only carry 12 clubs because there are no real "gaps" between my current set up. I use two hybrids but my longest regular iron is a strong 7 and carry 3 wedges where i want to minimize gaps. We can say get the 21 hybrid or we can say get the 23 hybrid but until you hit it with your shaft selection and your swing, we have no way to know if it was the right one for what you need. Hope this helps. If not, just ignore it.

With respect, I worked with my fitter when setting up my clubs. In my set I have a (917D2) 11.25, 917F 14.25, (816h) 18 and (818) 21.75 degree hybrids, then T300 5- W53 irons, with a 58 degree Vokey......took me a while to get the right set up but I am rather happy with the yardages.......I use the 21.75 rather than a 3 or 4 iron.....at my age, 73, am not a long hitter, consistency and accuracy are important to me, which make me manage my shots more carefully.
Less "go for's" then when I was younger. Think you just have to play around until you find what you feel comfortable with. I have played only one round since the first week of December so I do not know what to expect when I return full time to the game. That round was not the prettiest. Cheers and the very best of luck. =)

Gapping is really key to consistent distance and hence better performance.In terms of fairway vs hybrid generally speaking hybrid shafts are a little shorter than fairway wood shafts It may come down to personal preference as well as what the numbers tell you.