ETH implements a large size data set for your GPU to mine, so for this it uses your GPU's Vram. As the number of blocks increases, so does DAG size. Your Vram has to be higher than current DAG size to mine. It doesn't matters if GPU is specially made for mining or not if it has low Vram. Currently DAG size is 2.7GB which means you still can mine with 3Gb Vram GPU for a very short time, so 4GB Vram and models which you stated, are still usable and will be usable but can't say exactly for how much time.
Here is DAG size calculator to monitor yourself:
https://investoon.com/tools/dag_sizeSincere apologies if I would sound like a newbie, but based on both your comments, does that mean that VRAM size is only relevant in relations to ETH or EtHash mining? What about other algorithms?
Oh man, no problem, you are welcome!
Yes, VRAM size is only relevant in relations to coins based on Ethash algorithm. You can even see that information on website which I already posted, here it's called DAG (Directed Acyclic Graph), it's current size is 2.71GB as for now, when I posted it was 2.70
The models you stated above will last for 2-3 years, you'll roi at that time and possibly sell them. For mining profitability, you can check whattomine.com and choose GPUs based on it, see GPU reviews for mining on 1stminingrig.com. Buy GPUs with Vram 4GB or higher. Btw 6GB Vram and up is ideal.