Let's consider each law seperatey.
1) and 2): Gauss law for electricity and magnetism:
Its integral form was first formulated by Joseph-Louis Lagrange in 1773 (See here).
The next step, the Gauss divergence theorem, was also at first formulated by Joseph-Louis Lagrange in 1762 (See here)
However, it may seem strange, that the credit to both the above equations is given to Gauss.
3) Maxwell-Faraday equation:
This law is about electromagnetic induction which was discovered independently by both Faraday in 1831 and Joseph Henry in 1832. Faraday explained electromagnetic induction using a concept he called lines of force. However, all scientists at that time rejected him. Then Maxwell came along and mathematically expressed Faraday's theoretical ideas. Thus this law of electromagnetic induction can be righteously called Maxwell-Faraday law.
It states that, whenever magnetic flux linked with a circuit changes then an induced electromotive force (emf) equal to the rate of change of magnetic flux is set up in the circuit.
4) Ampere's circuital law:
Historically, there are two forms for this law. The one without displacement current term and the other with it. Both were introduced into physics for the first time by Maxwell in 1855 and 1861 respectively. Ampere has nothing to do with this law. I strongly believe if Ampere was alive at the time Maxwell presented this law, he will not accept it as it was opposed to his action at a distance Newtonian way of explaining electrodynamic phenomena. Anyway, it is apparent that why Maxwell is credited for this law.
From the above discussion we can only say that Maxwell could only be credited for two equations but not for all four. Most probably it was entirely due to Oliver Heaviside that these equations got their name. But there is a reason on why Maxwell is credited for these. In his 1865 paper "A Dynamical Theory of the Electromagnetic Field", for the first time using field concept, he used these four equations to derive the electromagnetic wave equation.
Thus these four equations bear and should bear Maxwell's name.