I refuse on ethical grounds!
We're all stuck on level 4...I've tried every combination of multiple differences between the cards themselves (you spelled 'difference' wrong in the title-unless this is intentional? You also didn't fully punctuate your comment in the source either...unless this is intentional too? I would have thought 'card position' was the solution too), cryptic solutions (like 'no differences', or 'nothing different', 'next level', 'user password', etc.), the cards spelling things out, differences between the kings represented by the cards, etc. I have this sneaking suspicion that I have entered the solution allready, just in an incorrect form (like a plural noun instead of a singular). Though I don't want the answer from you Maggot, I'll plow my way through it.
Does the game tell you if you get one of the criteria correct, or does it display 'incorrect user/password' every time?