These days your 4gb cards are worthless, even ETH needs 5gb today, and 6gb tomorrow;

You can't upgrade cards, you must sell them to a fool/sucker, and find some new 6gb cards, but you'll not find them because the stock is now world-wide empty for 2-3 years in the future.

U can hack BTC with these cards, see 'vanity-search' on git-hub,  and/or run bit-cracker, and hack it to make it run faster ( add a bloom filter to the code ), these algo's to hack BTC on GPU only require 2GB so your good.

BTC POW is called "MINING" its done on S-19 Bitmain ASIC systems that cost $5k and their all in CHINA, and all mining ( block-chain validation ) is done in CHINA.

I would teach you how to upgrade 'vanity-search' to hack bitcoin, but everytime I post links on the subject the people on this board on the 'technical' delete the posts, as its clear that this forum doesn't want people to know how to hack bitcoin for real.

But I can tell you here how to do, and I just did above. To get started you just pull the bloom-filter out of the old brain-flayer, and you add the bloom-filter C++ to the vanity-serarch GPU engine, that way on a GTx-1060-4gb card, you can do 200MB/H/Sec, 4 cards will do 1Billion/H/sec, with the bloom then each cycle is 300M btc addresses being compared,  so your effective is 300Billion-Hash/sec

There are no rewards in the bitcoin world, there are in the alt-coin world, lots but most have to do with running nodes, and nana to do with mining

These days most mining is done on ASIC macines built in CHINA, for all alt-coin & BTC

There is no such thing as a 'validator', as that is what miners do when they add transactions to the block-chain ( done in China )

...

In summary if you want ETH, then sell those cards and find 6gb cards, if you want BTC then hack gpu tools from git-hub to find 'lost' bitcoins you can use the GPUS you have.

I can think no other way to make 'money' from old GPUS with les than 6gb than hacking BTC.