Saliva, urine & CM/semen all have very different ranges to be considered 'normal' pHs. Are you testing saliva correctly? It is normal to run lower than urine, but 4 is definitely really low. It makes me think maybe it was too close to to eating/drinking/taking supplements and maybe is inaccurate.
Also, remind your DH that a temporarily low pH isn't going to kill him. It's not like he's on this diet for life!