As the 4 iron has around 24 degrees of loft i would be looking to put another club in the bag rather than mess with the lofts

an other hybrid of say 21 degrees ,  

I  do love my two iron though ? anywhere up to 230 yards and a make or break shot maker in the wind .

Gave up on the long irons and went with the Titleist hybrids.  Best decision I even made.  Carry three:  16.25 (913 17h), 19.50 (915 18h) and 23.25 (913 24h) then 714 AP2 5iron thru PW,  SM4 50.08,  SM5 56.10.  Of course at 69, need all the help I can get.  Much easier to hit.  Go to a demo day and check them out.    

Depending on how many clubs you're currently carrying, you could always try bridging the the gap with 19 or 21 degree hybrid as well.  I'd recommend seeing a fitter at a demo day in your area.  They know their clubs and would have a great solution for you as well.  I had a blast at my fitting and I found that I needed a different shaft to help control the distance better.  Hope this helps and best of luck to you James!

I had the same issue in the past and since the standard gaps on the AP2's start at 4 degrees and then between the 6 and 5 irons switch to 3 degrees. I adjusted the loft on my 5 iron 1 degree to 26 and my 4 iron 2 degree's to 22 to go with my 18 degree 913HD 

This setup worked well for me until I finally switched out the 4 iron for a 23 degree 913HD 

Great question - I had the same problem at the top of my bag.  How big is the gap?  I ended up adding two hybrids and ousting the 3 and 4 irons. Since the 913/915 hybrids are adjustable, you can really dial in to your swing, distance.  The only other question I'd ask is are you looking for a club to use for multiple shots or mainly off of the tee?  You could always look into the 712U (hybrid driving irons) as these come in several lofts as well (2-4 iron) if youre looking for some versatility off of the tee.  Its really about two things, the shots you're looking to hit and how big of a gap you need to close.

As others have said, sounds like adding a club (if you can) would be the best solution.

Good luck and keep us posted!

Rich