I heard someone that they can still mine with 4gb cards without an 8gb card by switching to Phoenix miner and use Linux OS as it use less DAG Epoch.
Look at this log that I get from SMOS.
phoenixminer-v4.9b (Added support for AMD RX5500. Fixed problem with loading NVML library with the latest Nvidia drivers. Fixed problem introduced
RX5500 has 4gb memory and it works with phoenix v4.9b so my guess is if you use phoenixminer the latest beta version of phoenixminer 4gb cards will work.
Can you try to use SMOS without 8gb card?