The number of players, less one, must divide into 3 or 4 with no remainder.
If a number is a multiple of both 3 and 4, it must also be a multiple of 12. This may not be instantly obvious. If you can look at the remainder of divisions by 3 and 4, the remainder 0/4 is 0, of 1/4 is 1, so the remainders of 0,1,2,3,4,5,6,7,8 when divided by 4 go
0,1,2,3,0,1,2,3,0..
Similarly for 3. -
0,1,2,0,1,2,0,1,2,..
The only time the remainder is zero in both cases is for multiples of 12.
It is not true to say this for all numbers though- eg if you want a number that divides exactly into 3 and 12, it's multiples of 12 again, not multiples of 3*12=36. Look up prime factorisations...