I think you still need the whole blockchain before you can prune it.. So I don't think you can use the 4GB but just use an external HD.
I don't know about setting it up on a RP0 because I've only set mine up on RP3 but this is a really great full node tutorial:
Damian Mee's RP3 Full Node Tutorial
I used this one for setting up a lightning full node on RP3 and if you like this one better you can use it for the regular full node just stop before the lightning part.