At first, You posted 3.99 I though it was in dKH but the value is in meg/L , so convert it to dKH is 11.2 which is ok. Try test more frequent to see how your Alk swing in a course of day. It might shock the coral by Alk swing too much.
You think the swings can be that severe? Do you think they are swinging up or down? I guess I'll find out soon enough. When do you suggest doing the tests?