How to calculate the thermal rise of the MOSFET?
Q1) Yes you can, if we assume the mounting base to be at 50°C, then using 4 W dissipation and 0.55 K/W (max value) thermal resistance of junction to base we get:

Tj = 50°C + (4 W * 0.55 °C/W) = 52.2°C

Q2) Yes, the method of calculation is the same as above and the "starting point" will be the ambient temperature instead of the mounting base temperature and you simply add up the thermal resistances:

Tj = Ta + Pd * (Rth_amb_to_base + Rth_base_to_juntion)