Ivanchuk plays 4.a3

This move prepares 5.Nc3 and then 6.Bmoves since if black takes on b2 his queen gets trapped with 7.Na4.

I thought this was good to mention since this is a standard tactic that is good to know in a variety of openings.

I appreciate that move orders are critical at this level. Nevertheless, if my preferred plan is Nc3, why not play it first. it seems to give more options and coverage than a3. I could then play a3 next move to help develop the bishop if needed.

Is there a threat of c4 or b4 or similar by playing a3 first?