Rachmaninov 4th is not too rare. The original version was performed in Cincinnati two years ago, for instance. If you want to learn it, stick with the original or 1927 versions. There have been too many recordings of the butchered final "revision". I'm certain that, if you learn the original or 1927 version, you'll be able to find concert halls to perform the piece in. After all, each of them has only been recorded once. People are tired of the 2nd and 3rd concertos, and the 4th (original or 1927) is a wonderful piece.
I think it's wise to learn a mixture of obscure and common pieces. That way, you're more likely to get to record (obscure) and perform (common). Who needs another recording of Tchaikovsky's 1st concerto? What we need are more recordings of Tcherepnin's 5th concerto, Rachmaninov's 4th (original or 1927), Tviett's 1st, Scriabin's concerto, etc. Scriabin's concerto is well represented by two recordings already, so it's not terribly obscure.
Obscure repetoire of the highest quality isn't easy to come by, in the piano concerto realm. But, obscure piano sonatas of high quality are relatively easy to come by, especially those of the early 20th century. Myaskovsky's 2nd and 3rd. Roslavets' 5th. Ornstein's (unfortunately, never written down or recorded... so not so easy to come by!).
A lot of obscure repetoire is banal, which is why it never caught on. That's not true in all cases, though. Roslavets wrote some outstanding pieces, but was forgotton by the music world because of Soviet politics. Myaskovsky's early sonatas became forgotton for the same reason.