I-140 approved, change of status to H4B

[kk77]

I am on H-1B, I-140 about to be filed premium processing. I might have to go to H4B to take care of my new born son. After I-140 is approved, can I go on H-4B and still retain my priority date when I file I-140 through a new employer ( say 6 months later).
Plz Plz help, my son needs me.