Where are you in Washington that it's Zone 4? Anyways, I had russian Kale last year. I cut it down and removed it after it bolted in the Spring, but it lasted through the whole winter without any protection. Once it gets cold and frosty, it will sweeten again. There is nothing wrong with starting over, but if it hasn't bolted yet, there is nothing wrong with leaving it.I have 8 rows 80 feet long by 2 to 3 feet wide. I will be planting a bunch more Arugula and Kale this week. :) (Not 80 feet worth though haha)

For my kale, I just take a leaf here and there and it keeps producing more. I never harvest a whole head at a time. I would just allow it to continue and harvest what you need when you want it or start harvesting for the freezer. Red Russian seems to withstand the heat very well but my dwarf Siberian did not even bat an eye when it was hot here.

I live in the mountains north of Spokane, east of the Cascades. We can get -25 in the winter, and about 20-25" of rain, a lot less than western WA. Some zone 5 plants survive, others don't. I've had some plants labeled as zone 4 not survive a winter, but then again, a couple labeled zone 6 that are doing well (these are rare).

either way -

cut stalk back & it will resprout (or snip leaves off bottom leaving at least 4 leaves on top & let the stalk stand tall then plant lettuces in the shade around it)
or

leave it to go to seed & let it plant your fall crop
(wait until the pods are thick & dried before cutting back - you can open them & plant yourself or let them disperse -- either way works for me with Red Russian Kale. You can lay the stalks down on the soil when seed is ready & it shades the new seedlings when they sprout.

I've let it plant itself for a few years & move the seedlings to spread them out a bit. They transplant well when young. May flop a bit, but perk up if kept moist & partially shaded in heat of summer. Sometimes I put an upturned pot or slurpee cup over them (or a cardboard box works, too.)