It means that when you select the steam option, the pump produces steam at up to 4 bar. which is more than enough
9 bar is the optimal pump pressure for pouring the shot.
On a related note, after purging my Piccolo (OK, no innuendo, cappiche?) I find if I wait until the steam pressure builds up to 4 bar (or more) Im going to get bubbles. What is the optimal steam pressure for milk texturing? From experience just over 2 bars seems good or does it depend on teh machine?