If you buy a new GPU every three years or less and want to sell more easily the old card, you want 8GB.
If you bought a GTX 1070 or 1080, well, you already have 8GB.
If you think you want to crossfire an RX 470 or 480 down the road instead of upgrading to a new card, 8GB (in each card) will be better.
If you game on 1080p or 1440p and you're absolutely sure you're not going to buy a new monitor any time soon, and/or never use your GPU to do adult tasks (like video rendering), and/or also if you never sell your old cards, and/or you really would feel bad spending four tasty hamburgers to have 8GB instead of 4GB, then you may consider 4GB.