That is a solid plan to have 121 time on your resume. Keep in mind though that airlines still value 121 PIC time far more than that of 121 SIC, JB especially. Will they hire FOs, of course, but having 121 PIC time will certainly help. Your chances of upgrade at XJT in the next 4 years are slim to none, therefore I would go to an airline like Endeavor that has the ATL base and guaranteed movement where you could potentially upgrade in less than 4 years and put yourself in a much better position to be selected by another airline. Again, I cannot recommend XJT to anyone because of a very uncertain future.