When I approach a fence in my head I say one, two, one, two, three, two, one and then we jump. I start counting one, two somewhere around 6 or 8 strides out to get the rhythm I want. This worked really well for me until I started seeing the occasional four strides. I then started getting confused, thinking there were three strides when I actually saw four (usually the result was me running) or thinking there were four when there were only three (causing me to ride backwards). I talked to Sue Berrill about it and she had said she went through a period like this as well. It made me feel better to not be the only one that has had this problem, but I wasn't quite as confident seeing my distance as I had been.
The way I got through it was just trial and error, with lots of poles and hoof prints when I wasn't jumping. I just started to make myself realize when I was far enough out that it was in fact four strides. This phase lasted the entire spring, summer, and fall. I didn't often bring it up, I didn't want to make excuses, I think it was chocked up to me missing the distance (which I was). It wasn't something anyone could help me with it was just something I had to figure out on my own. This fall I didn't jump any of the horses for about two months, they all had time off and when we started jumping this winter, I all of a sudden had a reliable four stride eye.
Now I am not saying I can see four strides and make the distance be perfect every jump, but I am much more reliably seeing the four strides and changing the horses canter to make the distance fit. I never dreamed I'd have a four stride eye, I was so happy when I finally had a pretty good three stride eye. I have to say though, seeing the distance one stride sooner gives you, what seems like, so much more time to fix the canter and change the stride if needed.
So, moral of the story, you can develop a three or four stride eye if you practice! It is a lot about the quality of your horses canter and how rideable they are so that you CAN make changes when needed. If you work at it all the time, even when you are doing flat work (using hoof prints) you will get better!