Almost any design can be done either larger or smaller than the original. You just have to figure out how many of the individual units you would need to make in order for it to be the finished size you want. I find graph paper/
quadrille paper very helpful when designing my own or redesigning an existing pattern.
In your case the 3" units would end with a finished design 3/4 the size of the original that called for 4" units.
Hope I've made sense!