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....
Well , i can sell 4gb GPU for $80-90. How many of it a have to sell to by one 5700XT

? So certainly il stay on 4Gbs and will mine whatever i can.
I had the same issue as you, I basically sold some of my 4GB models and added a few bucks and got the RX 470/570 8GB models instead. I looked at the newer GPUs or Nvidia and honestly its not worth it considering the long amount of ROI time you need to wait.
Most of the 8GB cards (except this month) were maybe $30-50 more than the 4GB models. So if you sold 5 of your 4GB cards you could of had an entire rig with 8GB models for only $200 more, rather than spending $500 on the latest and greatest GPU, not worth it.
In Dec when 4GB GPUs can no longer mine the difficulty will be half and it will pay itself off then. So the investment is worth it.