What is the proper left-hand fingering for the chord, Bb(sus4), in root position on piano?

Answer 1

The sus chord is generally assumed to include a 7th and has a dominant function so:

Bb - Eb-F-Ab would best be played with 5-3-2-1

Without the 7th, (the Ab) 5-2-1 -- but this robs the chord of some of its color

Often it is played without the 5th (the F in this case) so 5-2-1
But this takes the dissonant bite out of it.

Answer 2

5 - 3 - 2 - 1