You did not say what type of irons that you are playing, so I will assume, which I hate to do, that you are playing something similar to the AP1s, which the 3 iron is a 20* and the 4 iron is a 23*.....Titleist makes two types of hybrids.  913H and 913H.d.....in the H model you could get a 19* and a 21* and in the H.d model (lower spin) you could get a 20* and a 23*.......if you want to replace them with two new clubs.....Titleist hybrids are excellent clubs.....I would recommend getting fitted for these clubs.....you want to ensure that you have the correct shaft.....the most important part of the club.....good luck....... 

I went with a 24* to replace my 4 iron and a 19* to replace my 3 iron and 5 wood.  Was able to put an extra wedge in the bag by doing that.  If you just want to replace the 3 iron go with a 21*.

Jerry,

I had the same dilema and considered two options, the first was to go with the 18 and 23 degree hybrids as one setup for my bag.  I also have a 4 iron and 20 degree set up for an alternative.  Each gives me a slightly different distance coverage depending on the course I am playing.  If you wanted to exactly replace your 3 and 4 iron you would consider the 20 and 23 degree hybrids but I think they are too close and you may wish to consider a slightly larger spread.  I use the 913 Hd line of Hybrids vs the traditional 913 H line, FWIW.

Good Luck!

I will second Chris's 18 and 23 recommendation.  That's very similar to what I did and I've been very happy.  My distance gaps are very good and I find those two hybrids will work for just about any shot I need from long range.