My fingers could never be described as long and slender but I can and do cover 4 frets.

It may upset classical guitar teachers, but the thumb is far from irrelevant when used on the 6th string. It avoids all those awkward bar chords and leaves the first finger free for picking out the melody. I've even seen someone who managed to use his thumb on both the 5th and 6th strings.

I can play sitting down if I am slouched in a chair, in some very unorthodox position, but if I am sitting straight up then I prefer to use a strap, so it is much the same as playing standing up.

DC