The four different comitia were the supreme assemblies of the Roman people as a political body. The comitia were the legislative power of the Roman state where Roman citizens voted laws and elected magistrates.
Relationship between the senate, the magistrates, and the comitia
In antiquity there were accepted conventions about the relationship between the senate, the magistrates, and the assemblies. They helped to make that three-sided relationship work smoothly. However, it's important to remember that we are not talking about a direct relationship between the senate and the assemblies. There was no such relationship: the senate and the assemblies had no direct dealings with one another at all. The magistrates were the intermediaries. The senate gave advice to the magistrates, and the magistrates put proposals to the assemblies. The senate could not put proposals to the assemblies but only advise magistrates to do so.
In general it was considered proper for magistrates to take the advice of the senate before bringing legislative proposals (rogationes) before the assembly. This was not compulsory, and failure to consult the senate did not invalidate a proposal put before the assembly. It was, however, a very strong tradition, and a tribunus plebis would often veto a proposal which had not previously been discussed by the senate.
There were, however, exceptions. Certain types of proposal were understood to be within the exclusive competence of the assembly, and if a magistrate brought such a proposal to the assembly without consulting the senate, he was not considered to have behaved improperly; but equally he would not have been accused of impropriety if he had consulted the senate first. A tribune could not use failure to consult the senate as a reason to veto a proposal of this kind.
Consulting the senate on the merits of a proposal should not be confused with the patrum auctoritas. The patrum auctoritas was a technical device: it was rubber-stamping, rather like the Queen approving Acts of Parliament in the U.K. It was quite separate from any discussion by the senate of whether a proposal was actually a good idea or not.