kslider: how to limit it ?

You'll have to precede it with a little logic. You can count how many note ons have been received and ignore further note ons until you get some note offs, or you could use a [zl queue] and make the fourth note on "pop" the first one from the buffer, reconstruct it into a note off, and send that message to the kslider. It just depends how you want it to behave.