finaly we should sell our 4GB graphic cards or we can mine ETH in future?
This is good idea , but how can you sell 50+ or 100+ those GPUs ?
for its weight ?

Ive switched for 8Gb in a couple of rigs but its so hard to find more or less convinient deal

Can mine Ravencoin, or sell and then buy used 8gb cards.... or if high power cost get 5700XT??? Vega cards can get 50MH, so also good if get good price on them. You can use Phoenixminer to still mine (not good with the display card though...) on 4gb cards, and use claymore to still set -rxboost (set bogus pool on claymore......), or set the REF param with amdmemorytweak....