sp⁴ Hybridisation is not possible. And simple reason is that p-subshell has only 3 orbitals. If we take sp⁴ into consideration, then what would have been the consequences that prove that it is impossible to have sp⁴ .

Then you have to build the 4th orbital which is impossible because you can`t stabilize this since this orbital favour for another to form a pi bond which come out to be another problem as in case of bond between atoms in NH molecule and pi bond in case of benzene derivatives. Nature tends to be in stability u can`t doubt it but can solve it by studing instability of intermediate formed during these natural process but the goal is all things favour stable at the end if not then the examining is incomplete and question raised can`t be answered properly