How to create continuously incrementing counter in structural modelling?
An AND reduction of 4 bits can be performed using a 4-input AND gate, and is consistent with your intention of writing structural code.

Having said that, the block that you call counter isn't a counter at all...it is just a 4-bit adder. To make it behave as a counter you need to add a 4-bit register that will load the output of the adder on every clock edge. The input to the adder then comes from the output of the register.