A magwell widens and chamfers the magazine well so the incoming reload is funnelled into place rather than threaded in precisely. Under stage pressure that larger entry makes reloads more repeatable, and more repeatable reloads mean less time lost between strings.
The M9A4 follows M9A3 ergonomics and may share its lower-frame geometry, and a compatible base pad closes the funnel at the bottom of the magazine. Confirm M9A4-specific magwell and base pad compatibility on the individual product page before ordering.