Ok, here are some observations after using X4 for not even 24 hours yet. I have used Captcha Breaker and Captcha Sniper X3 as well so I'll try to compare a bit. I use it mainly with GSA SER so that is what this info comes from.
First off, CS X4 is the fastest captcha solving software I have seen. After attempting over 80,000 captchas my average solve time is 0.124. With Captcha Breaker it was around .250. I don't know if this is a big enough deal to make a huge difference yet, but CS X4 wins on speed. My GSA campaigns are very well optimized and I was able to get over 200 LPM at times with CS X4. With Captcha Breaker I hit above 180 at times. The difference is too small to really make a judgement on a small sample.
Next CS X4 seems to solve more catpchas than X3, however not as many more as I was hoping for. I may have just set my expectations too high after reading the marketing for it.

But it does solve more. I would say it's pretty even on solved captchas between CS and CB again with the limited numbers I have so far.
One great thing in CS X4 is the interface and especially the captcha destruction kit are GREATLY improved. Between this and the speed increase I believe these are the best new features. Captcha Breaker's SDK is good too so this is really a toss up.
One thing Captcha Breaker edges out CS X4 on is their updates occur several times a week. I see an update button in CS X4 but it gives a server can't be found error at this time. If Captcha Sniper can get more frequent updates going I think that would be a huge boon for them.
Really Captcha Sniper X4 and Captcha Breaker are both great pieces of software I don't think you could go wrong with either of them. Both are well supported by their developers with fast responses to problems. They solve a TON of captchas in GSA SER and will save you a lot of money. If anyone else could comment on their performance in other programs like SeNukeXCr, Licorne AIO, etc I would like to hear what others are getting.