I agree that Harding's commercial recording is disappointing. There is a pirate from a performance of M4 that he did in Leipzig (Gewandhaus Orchestra) that's far, far better. If you'd like a copy of that, leave me a private message. I don't know who the soprano was, but she was a definite improvement. It's really quite good.
As far as instrumentation for M4 goes, Mahler makes no mention of reducing the size of the string section. There are no trombones or tuba. The brass are only four horns and three trumpets. Mahler calls for only three each of the woodwinds, with the exception of four flutes (parts 1 & 2 double on piccolo). The high Eb clarinet writing is included in the second clarinet part, and the 3rd clarinet part has the bass clarinet writing included. My guess is that it's common to have a fourth clarinetist on hand. The contrabassoon writing is included in the 3rd bassoon part. In my opinion, you can never have too many bassoon players. The English Horn (Cor Anglais) writing is included in the third oboe part. Only one set of timpani is involved (the first three symponies require two sets of timpani each). However, you need at least four utility percussionists. In the first movement, leading up to the climax of the development section, you need five separate players to cover timpani; glockenspiel; triangle; tam-tam (gong), and bass drum. In non-professional orchestras, if necessary, you could probably teach one of the string players to cover one of those utility parts, without too much hastle. There's no indication to double the harp part (only one part). However, I think it's not uncommon to have two harps on board. Still, this is a lot of less instrumentation than what Mahler calls for in his first three symphonies.
One bit of oddness in terms of the instrumentation: for the second movement's solo violin part, it's common practice for the principle violinist to have a second violin on hand, that's already been tuned one step higher (as the part calls for).
