After 1NT, 11+ 4-card Major
Most intermediate and advanced players use Stayman in this position. Stayman allows you to find a 8-card Major fit if there is one, or revert to NT if there isn't. And whichever is best, it also allows you to find out if you should bid up to Game.
Stayman with a flat '4333' hand ?
One of the benefits of using Stayman to find an 8-card Major fit is that you'll be able to ruff your short suits in each hand, thereby creating additional winnable tricks. But if your hand is a 4333 shape, then your 4 trumps can't be used at all for ruffing, since you don't have any short suits.
Don't use Stayman as a NT response when you have a flat '4333' hand, you'll be better off in NT.
Instead, or if in any case you haven't agreed with your partner to use Stayman to explore your team's options, then here's what you do. . .
Balanced
With a 4-card Major, but not a 5-card one, in many cases you are going to be balanced.
Since you have no 5+ card majors, your distribution is probably 4-4-3-2, or 4-3-3-3, or 4-3-5-2, for example. In that case just go and look at the last << page.
Unbalanced
With the rarer distributions, 4-4-5-1, 4-4-6-0, 4-3-6-1 you are obviously unbalanced. In that case just go and look on the next >> page