what is the difference between QPSK and 4QAM?
There are different types of QPSK: Offset (staggerd) QPSK, $\pi/4$-QPSK, etc.

In its most known form, QPSK is identical to $4$-QAM or $4$-PSK. That is, the same constellation can be referred to by different names. So there is no difference.

This constellation can be seen in the picture bellow (from Wikipedia)



Similarly, $2$-QAM is actually the same as BPSK, as is also pointed out here.