I just did a bit of web searching, and I now believe that "ET27" refers to an offset of 27mm or 1-1/16". Given the width of 4.5", I think that for most steel wheels that would correspond to a backspacing of 3.81".

If that is so, now all you need is the bolt circle. The best bet is to measure it as I wrote earlier. Although, if it is a European glider trailer, I would put my money on 4-on-108mm.

Thanks again, Bob K.