Indeed.
Having an inkling of how much time and effort Rigo put into making AmiUpdate work with OS4 again, I would like to publically add my thanks for this.

@bamiga2002

I have been discussing the same issue with Rigo, who tells me this is because update.library in turn looks to open several other v53 libraries.
OS4 Classic has v52 libraries, so it fails.

Rigo will investigate if dropping the version number will cause any problems, so you may (or may not if its problematic) see an update to allow operation on OS4 Classic.