Numeric Stepper

is there a way to make a numeric stepper that starts from 5, with stepSize 4?
i.e giving this:[AS]on (load) {
this.value = 5;
this.stepSize = 4;
}[/AS]
seems to jumps from 5 to 8 -> 16 -> …, instead of 5 -> 9 -> 13 -> …
(I’m smelling a bug because when you ask the nextValue of it, it gives you the right value)

any ideas on how to solve this (maybe using a prototype?)