Before his/her 485 is approved, he/she can come back on H4. After his/her 485 is approved, he/she can only come back on valid AP. At the time he/she left for China, CIS has no idea if he/she left on AP or not. However, if she/he stays very long outside the US, CIS officer may start doubt his/her PR intent, thus causing problem on his/her 485.