Hi Fubin,
It doesn't matter. You can left it floating.
PTA4 is default as NMI_b/EZP_CS_b function.
If you don't need EzPort and Non-maskable Interrupt function, an external 4.7 kΩ to 10 kΩ pullup is recommended.
Or you can disable these function by using FOPT[NMI_DIS] bit and FOPT[EZPORT_DIS] bit.
Best Regards,
Robin