The use of a bid in the 4th suit by responder at this second turn, as an artificial forcing bid. The 4th suit doesn't promises anything in the suit but tells partner we shall not stop under game level. Someone us it as just invitational strength but I think GF is best.

The normal response to 4th is to threat it as a question for a stopper in the suit, and if you haven't, you continue showing distribution.

Many use the sequence 1C-1D-1H-1S* as just round forcing, were 1S is artificial, but opener support spade as it was natural. 

Advance use of 4th suit
You can make out rules for how you respond to 4th suit. One way is to show distribution if 4th suit is at 2-level, and show stopper if 3-level. So if 4th suit is at the 2-level, you respond like this:

You may also define that if 4th suit is at the 3-level, 2NT is GF asking for distribution and 4th suit is asking for stopper. This is a very flexible way to use it. You loose the chance to play 2NT, but it is not that important.