I think we can. It sounds like your shifter isn't indexed at all (has multiple clicks for each position), so you could stick any freewheel you wanted back there. If you didn't mind that the numbers don't always line up with a particular gear, you could get a wheel with a 6- or 7-speed freewheel (much easier to come by and in decent quality) and enjoy a few more ratios in between the ones you use.