Four-vector Momentum problem

Hey, here's a question im not too sure about.

An electron and a positron travel on opposite directions along the z-axis (with the electron travelling in the +z direction) and collide head-on. Their energies are Ee and Ep respectively, and they both have mass m.

a) Write down their four vector momenta in the lab frame in terms of Ee, Ep and m.

The electron and positron fuse briefly to form a single particle, labelled X. For the case when Ee = 50 GeV, Ep = 100 GeV and m is negligible, calculate the four-vector momentum of X i the lab frame, and its rest mass.

How would these be written as vectors and why?

How do you convert between rest mass and relativistic mass?