Yey very common. Dont go chasing the ph. Unless the plants tell you otherwise. Mix your nutes. Let them stablize a couple hours adjust the ph then change your rez. The ph will swing up and down with a healthy feeding plant. Drastic swings can be a sign of root problems ( ph 4 is a very drastic swing) again though your plants will let you know if there is a problem as long as your listning.
Better under feed than over feed. The plants look under fed? Nope? Then dont give them more yet. Keep good notes. When they do look under fed give it to them and make a note and next time you can up there feeding just before they show signs