The irony here is that hitting the keys softly is good technique. But it is hard to do properly; particularly when you are faced with anything requiring lots of speed. The reason that hitting softly is good is because when you hit hard you generally tense up, and end up causing both muscles to contract at once (the muscles for extending and for retracting your finger). Doing this slows you down and strains your muscles.
Personally, I find it easier to play with my fingers less curled and a bit more flat, which runs completely counter to what abraker says. I found that with more curled fingers, I had problems with stamina and with hitting harder. I play 7k mainly, to make it clear, which does overall require less speed per finger, but I would be quite surprised if hitting lightly was not also good technique for 4k. I actually made an adjustment to my key settings for my right hand to make my fingers a bit flatter. I moved from sdf kl; to sdf op[, which had a pretty noticeable impact almost immediately.
I have had my fingers lock up and just basically refuse to move, but that generally only happened when I was trying to force myself to play as lightly as possible. If you're finding your fingers are locking up, just try not to hit quite that lightly.