zynex 4 digit pin gives 5040 combinations (210 if they should be unique), while 6 digit pin gives 151 200 combinations.
I'm not sure I can reproduce the calculations for four digits. If there are four digits, each 0 through 9, that gives 0000 through 9999, which is 10,000 combinations. Avoiding duplicates, 10x9x8x7 would be 5024. I'm not sure where 210 comes from?
But it is true that a six-digit PIN is waaaay better than a 4-digit PIN.