Why it is important to make a delayed raise and not jump directly to 4-lvl? (System: simple SAYC)
A common bridge principle for SAYC is to bid up strong hands slowly, but jump to a high bid quickly with weak hands that have long suits or other preemptive features.

In the situation you describe with responder having 13 points, you'll want bidding space to communicate whether or not you should be in game or slam, because opener has promised 13-21 points (and therefore the combined hands have 26-34 points).

If slam is possible, most tables will bid it up slowly and find the right slam contract, while your hasty 4 bid lands you a mere game contract (which will give you the bottom score in duplicate bridge).

Therefore, in SAYC, the jump to 4 of the opener's major response is reserved for a weak bid that typically indicates a small number of points but good shape.

For example, with 6 low trump, a void, and a single Ace for high card points, that would be a perfect hand to jump to the 4 level because:


You have a great chance of making the contract
You may prevent your opponents from exploring game
You would have no interest in exploring slam with such a hand, even
if partner has 20-21 points.