Is 4 hours to myself before the day begins sufficient for my respite?

I think the best answer is whether or not it works for you. Everyone needs a different amount of sleep to rest and recharge; some folks can get by on 4 - 6 hours of sleep a night.

You don't mention your age, or the stage of your husband's dementia, how many dogs you have, whether they're puppies or adults, whether you have children, etc.

If you feel rested in the morning, then I'd say you're getting enough sleep. If not, consider changing your patterns.

You're wise to address this issue now before things get more complicated and hectic.

BTW, you can use a photo of your Labs as an avatar. I always enjoy seeing animals next to posts!

Oh, yes! Many would envy you. I cared for mom a year before she passed. She couldn't walk unassisted, couldn't dress herself, wash...nothing. She would wake up at 6 in the morning clammoring to get out of bed . . . Try to climb out, rattle the railing. That was for the first six months. After that it was much better, and I would let her sleep until about 10 o'clock. That morning time was precious.