Hi
I also have this problem with my number 4.
I noticed it occured more often when i worked on end grain
It had to do with the way the iron lays on the frog if you check this and maybe flatten it it will probably improve.

Another thing that may help is to put some oil or wax on the bottom of the plane to reduce friction which also helped for me.

I hope this helps you a little bit

Cheers.

Daniël