I think it depends n what level you are at when you learn them. I mean, I think you would only get a fair assessment of the difficulty from a purely technical point of view if you learn them all at a time when you are capable of playing them all.
Most advanced students do a ballade at some point in their youth, and they usually do 1 or 3. I am sure, that at that time, that ballade is really hard, and pushes them to explore things that they have not explored. Then they will do another ballade, and probably find it hard, and then they will probably do the 4th at some point, and that will be really hard for them. So they can't really judge the difficulty, because they will all have been difficult. And by the time they play all 4, they will have been playing most of them for years. I don't know any pianist who just learnt all 4 straight away....but I think if there is a pianist here who did learn all 4 in one period, then he/she will be the best judge.
I did the 1st ballade first. At the time it pushed me in many ways, technically and musically. My teacher probably thought (and I also think) that it is the easiest to grasp musically. I then did the 3rd, but found that incredibly hard to play. To create the sounds and the elegance is very difficult, and it is a lot harder than the 1st in that respect...I would say that it requires a much more refined technique to play this. You need a very fine legato touch, and a huge range of colour. Then I did the 4th, because I was desperate to do it, and my teacher thought it would be wise to let me learn it when I was young. This was impossible for me. I did not have anywhere near the amount of control over sound that is required for this piece. I could hit all the notes, but it was really immature, and the last thing this piece needs is that! It is certainly the hardest technically I think, and this is easily discovered by reading through the score. I did the 2nd ballade last, and didn't find it too troublesome....the Coda is nasty, and that is all. Playing the slow sections is the hardest part, becasue they can become boring very easily.
From personal experience, I can only say that 4 is the hardest, and that I can't judge the other 3, becasue I have played then for a long time, and have no technical issues with them anoymore. They all have there problem passages, 1st-Coda, and some other awkward moments 2nd-coda- 3rd- Csharp minor section, and the 2nd last page.