Technically, two 4* players can already do this, as 5* ruins are mostly made up of Fire Spirits ( mercenaries ) so just team up to send a joint attack of mercenaries to take out the fire spirits and a second joint attack to clean up the rest with main armies.
I didn't say it would be easy for 4*'s but it's probably doable.
For the king, for the land, for the mountains
For the green valleys where dragons fly
For the glory, the power to win the black lord
I will search for the Emerald Sword!
Technically it can be done, yes. The main problem is, if you collect an army big enough to handle a lvl5 ruin, you step up to lvl5 yourself. I think even the joint forces of two lvl4 players won't be enough.