Hi there, that may make good sense but it's not entirely true (in the UK). If it were I would rarely be able to commence a drive at all as I am often in the 4s. A lot depends on an individuals circumstances of course (and whether they fall fast or not etc) but the Law states you CAN start off a drive in the 4s but should have a snack.
If you hypo whilst you are driving, then you MUST stop (obviously), treat the hypo and THEN wait until it's in the 5s to continue driving.