I have also used M4 Countersink washers in a pinch too... you're only gonna add an extra 2mm max of thread anyway, this gave me an extra 4mm overall track width increase which came in handy as a tuning adjustment on my front wheels to help add a little push.

It's amazing how many uses I get out of countersink washers, some come in 9mm widths which make excellent supports for mounting servos too: