I know our distances would most likely be different, but I'll share with you how the hybrids relate to my irons (AP2's reg flex).  

My 909H's play a minimum of one club longer than the same lofted iron.    The 19 deg hybrid covered the distance of 2 iron to 3 wood (when I had a 2 iron- 962's).  The 21deg 909H carries longer than my 3 iron AP2, so I use it for my 3 iron (when I choke up about 1/2") and the lack of 2 iron. 

That allowed me the option of choosing between carrying the 19 hybrid or carrying an additional wedge.  I opted for the additional wedge as I can always choke up on my 3 wood for the gap created by not carrying the 19.

In AP2's,  the 4 iron has a 24 deg loft.    So based on my experience with 909 hybrids, if I chose to replace my 4 iron I would choose the 24 deg.  

Make sure you check the specs on whatever iron brand and model you are playing and get as close as you can to the existing loft of your 4 iron.  That should be a safe bet. 

Good luck!!

It depends on the specific yardage required.  You can pretty much make any of the 910 hybrids dance.  :)  See if you can find an advanced fitter near you that can quantify distances on a launch monitor.  www.fittingworks.com.

RC

I tossed my 4 iron for a 24* 909 and I love it.  The beauty of the 910 line is you can adjust your loft.

Thanks guys.  I think I'm going to go with the 24*

I currently play the Titleist MB's in 4 iron through PW but I also own the 910H in 17*, 19*, 21* and 24* for different courses.  I also have the 910 D3 in 9.5* and the 910F in 15*.  A lot of clubs but this enables me to pick my clubs according to the course and the gaps I am trying to achieve.  The 24* hybrid has kicked out the 4 iron because I am able to get a lot more consistent yardage gap between my 5 iron and hybrid then the 4 iron.  I hit my 5 iron around 185 consistently but I was only hitting the 4 iron 192 yards so their was not a lot of gap space where is I hit the 4 hybrid around 200+/- depending on how I want to hit it.

The 4 hybrid, for me, is just way easier to hit then the 4 iron.  I never thought I would play 3 hybrids but they are so much easier to hit the yardages I am trying to achieve.

My distance control is so much better and I have a lot of different ways to go about my gaps depending on the conditions.  Heck, the 17* hybrid is getting close to knocking out my 910F fairway wood.  When the 17* is set neutral I only carry it 5-10 yards less than my fairway wood.  I am seriously thinking about making the 17* hybrid into a 16.25* and putting in the fairway shaft and see how the yardage difference is and if it is close replacing the 3 wood with the hybrid.  The hybrid is money from the fairway or the tee box and is just night and day easier to hit off tight lies going for par 5's in two.

Just my thoughts.

According to my fitting session, I ended up with a 17° 910H to replace my old DCI 962 2 iron. The 19° 910H carried as far as my MB 710 3 iron, and I wanted to keep a gap. It's nice to know, however, if the 17° ends up going too far then I can always adjust the loft up a bit to shorten it up.