I've always considered a 4 iron to be equivalent to a 24* hybrid.  However a couple weeks ago someone was posting that a pro told them a 3 iron is equivalent to a 19* hybrid..  I was shocked and still am, but according to them a 21* hybrid would be the correct choice for you.  I'm in the same boat as you are and looking to replace my 4 iron, but think the 24* would better suit my need.  You know what they say, "Go get fitted."

Since the loft angle varies from OEM to OEM, a 5I can be 25 to 28 degrees.  So a corresponding hybrid can be 21 to 24 degrees. What you want is a hybrid that fits into your clubs with an acceptable distance.  Fitting for an appropriate shaft and then selecting a loft to fill in your distance gaps is the second part.  Since the 910/913 H is adjustable, the 24 can be 24.75-22.5 and the 21 can set to be 21.75-19.5.  It just depends if you want/need an even gap between the 5I and a 5W (Titleist is 19 degrees, but this can be 17.5-19 degree from other OEMs.  The AP1 irons do have a lower loft by 1-2 degrees from the other 712 irons.

To correspond to other OEM hybrids, then a 21 degree 710H may be a closer match to other OEM 4 hybrids at least striclty on loft.

The loft and lie angle can be changed by +1.5* and  -.75* . The clubface at standard loft is .5* open and changing loft by .75* changes face angle by 1*.  I have my 19* fairway set up as a 7 wood (A3 setting 20.5 loft and 59.5 lie angle with face 1.5* closed). 

The standard loft for a "player" 3 iron is 21*.  AP1s differ from the player irons by 1* on the long iron end and 2* from the 6 iron down.  

Generally, the equivalent loft hybrid gets about 5-10 yards more vs the iron and the equivalent loft wood gets 5-10 yards more than the hybrid with some.  I know I'm capable of 200 with my 19 fairway but generally hit 190.  The best I've been able to get out of a 21 hybrid is 180 yards but I generally hit it 170-175; I tend to swing more conservative with a hybrid or iron.  There are also some that can hit a hybrid farther than a fairway wood.