Hi Steve,
With a 120 mm aperture scope, depending on your seeing, it might not be worth reducing all the way to f 4/5. In fact it might even be counter productive as your star image might be so much smaller than the slit width that it “rattles around” in the slit making it difficult to guide on the slit as there will be little or no overspill. It could also give potential wavelength calibration issues as the star location rather than the slit itself will start to define the spectrum position. I suggest measuring some star FWHM under typical seeing conditions first and comparing this with the ALPY slit width (23um). The optimum is to have the slit ~equal to the FWHM
Re the ALPY focal ratio, I think the aperture of the ALPY optics will accept an f4 beam but like most optics, they work better stopped down a bit. I suspect you will get a sharper spectrum and potentially squeeze a bit more resolution out at f5 compared with f4, particularly at the ends of the spectrum. This is certainly true of the LHIRES.
Cheers
Robin