Yesterday was one of those days for me, running 4-5 all day, but steady there until I dropped to 3.7. No problem at all, just have a snack and I was happily at 6.3 again.
Medically, 3.5 is a hypo as pointed out, but most DSNs would like you to be higher than that. DAFNE would like you to be >5.5 all the time. I was given targets of 4-6 in the morning and 4-8 rest of the time, >5 for driving. I like to keep in the bottom half of my range i.e. 4.5-6 as far as I can, but that's a personal preference as I'm not yet 40 and want to avoid complications over the next 30 years.
One thing to consider when getting down to 3.5-4 regularly is whether you feel it or not. If you routinely run down there it can reduce hypo awareness so that when you do go <3.5 you may not feel it, which could become dangerous.