I second and totally agree with Larry's suggestions. An R4 compressor should be an R4 compressor and "should" be a bolt-on swap (Fed. law required that a certain % of cars imported into US had certain accessories installed in the US - AC systems were one such accessory, so you probably have a US made/installed AC system). Look both compressors over very carefully (bolts/fasteners and how they fit, AC line fit, mounting holes line up, elec connections actually connect, clearances, etc.) BEFORE installing the replacement - not a good idea to be installing the replacement only to find something won't fit.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.