Dinah and Squiggy, both your ways would only work once (bugged).
This is due to you never reseting the variable to 0 after it reaches 4. Thus appon reaching a value of 4 once, it will never be that value again (well technically not as eventually it would interfear with the sign bit and loop around to a negitive number which would eventually be positive but no one ever does 4294967296 kills in one game).
What you probably mean to do is modulo the integer (getting its remainder from a division opperation). Thus in Squiggy's case it would be ModuloInteger(hiskillnumber,4)==0 which would only be true every 4 kills. Dinah, similary would be ModuloInteger(hiskillnumber,10)==0 which would be true only every 10 kills. This completly avoids the need for reseting a variable to 0 and thus enables it to be run from a general number of kills variable that might be used for other systems (like a multiboard).