Many players/systems differentiate a limit major raise with 3-card support versus a limit raise with 4-card support.
The question is, why would you want to do this? Suppose responder shows a limit major raise. With extra values, opener goes on to game; with a minimum, opener does not go to game. How does it help the opener to know that responder has 3-card versus 4-card support?
A Value (not a good reason)
To be sure, the fourth trump is a value. To be sure, you would be more encouraged to go to game knowing you will have 4 trumps in the dummy.
And this is the kernel of an answer. Suppose you make your limit raises with 10-11 HCP and typically an outside doubleton, and you do not assign any extra value to your fourth trump. Then your limit major raise with 4-card support is simply stronger than your limit major raise with 3-card support. Opener needs extra values to go to game opposite your limit major raise with 3-card support; opener needs only a minimum opening hand to go to game opposite your "limit" major raise with 4-card support.
However, the point of a limit raise is supposed to be that opener needs a little extra to go to game. You should properly evaluate your 4th trump. If your hand is strong enough to bid game opposite a minimum, force to game. Opener should need something extra to go to game opposite a limit major raise.
Bob sez: If you and your partner are more willing to go to game opposite a limit major raise when partner has 4 trumps, you are not adequately valuing your fourth trump.
It is absolutely true that knowing your partner has 4-card support can help you evaluate your trump holding. The best example is that the queen in AKQxxx is more valuable opposite 3-card support than 4-card support. The same can be said for lesser spots, though not quite with as much confidence. For example, the jack in KQJxxx is more valuable opposite 3-card support than 4-card support.
This is a pretty minor reason to differentiate limit major raises -- how often does this occur? And except for AKQxxx, how important is it? But if you are differentiating your limit major raises, you should at least use the information -- fillers and a chunky suit are more useful opposite partner's shortness, not length.
If you happen to have a 6-card suit, and you have the A and the K of trumps, and you are bidding slam, and you find out you are missing the queen of trumps, you know not to worry about it. Again, how often does this occur?
Part of the art in bidding is evaluating queens. For your queen to be valuable, your or your partner need to have an ace or king in the suit, and the queen is less valuable if you or your partner are short in the suit.
Differentiating 3-card support and 4-card support can help with this endeavor. The limit raise with 3-card support tends to have more HCP and less distribution. Therefore, the queen is more likely to be valuable. In contrast, the limit raise with 4-card support tends to be more distributional with fewer HCP. That means queens are less likely to be valuable.
So, when partner shows a limit-major raise with 4-card support, you are more interested in aces, kings, and distribution.
This too seems to be a marginal reason to differentiate limit major raises. But again, if you are going to differentiate 3-card support from 4-card support, you should use this information.
Some people also insist that a Jacoby 2NT response (to 1 of a major) should show 4 trump. Again, this is very useful information if the 4th trump is undervalued, of minor value when it is not. I think with some hands it helps people plan the hand better (for purposes of bidding slam), but I am not sure that is anything more than a psychological value.