Well yes of course at 1.25 - 1.3V terminal voltage 4 X 1.25 = 5V great for the 5V source but a poor idea on the DCin jack as the regulator will only have 4V3 on it's input terminal. The recommendation I usually make is 3 cells (Alkaline) on the 5V source and 5 cells on the DCin jack. It has always worked for me. However be most careful of the DCin connection on the shield connector as there is no protection there as there is on the DCin jack.
Bob