4x 2" stud partition with 1/2" slab either side= 5" overall, or in metric 100mm + 25mm= 125mm. Either way a 5" or 125mm door frame is what you need. (Be aware that a frame called up as 5" may be machined from 5" stock and may actually only be 4 1/2"! Your supplier can confirm the finished dimensions of any planed or machined timber stock you are buying.