I think you use 4GB cards and win10. win10 reverse about 20% VRAM, and the dag file size now 3.49 GB. 3.49+20% = 4.18 GB, so your cards doesnt have enough VRAM. win7 reserve less, and linux a lot less. If you change the operating system you can mine a couple of months before the same error.
Correct, and another method you can use is to use a 8GB gpu along with your 4GB gpus. Basically put it in the PCIe 0 slot which is the main x16 PCIe 3.0 slot (usually the highest x16 slot in the motherboard except for x1/x4 slots) and you can put the rest of your 4GB gpus in the other slots.
When you start to mine most of the Win10 resources will go to the 8GB GDDR and the remaining GPUs won't need that much space, and you can get away for many a few more months.
Either way, ETH is going ProgPOW and you can mine with 4GB again (maybe).
I tried this and it didnt work for me, I have 2 8g rx480 and 2 4g 570's. I just switched over to a linux os - minerstat for me - no issues and faster mining speeds.