And if I got it correctly, the next options would be:
1) Put 8GB card (as I have some in another rig) in the 1st PCIE slot
2) Switch to Linux OS (HiveOS for example) - no experience there, so I leave that as last resort.
Any clue what would be the best use of the 4GB cards after Dec, when the DAG exceeds 4GB?
Your first priority should be to get on Linux. These solutions couldn't be more simple (plug in usb stick, use hdd for your movies). Sell the 4gb cards while you can because their value is still ok and you can use the liquidity to get into cards for the future.