So what is the exact week/day, when 4GB cards will not be able to mine anymore?
Very difficult to answer. I will restrict to AMD 4Gs. Also limiting it to full speed mining:
On most Linux systems the last epoch is 381, some do 382, but that is rare. Some very rare cases are 380, but that is an old driver then.
Overall to get to 381 use a recently released miner that still gets updates and is tested for that. Often it will need a split DAG file (but e.g. Claymore still uses a single big DAG ... that may end earlier)
Epoch 381 by the way will last from Dec 15th to Dec 21st.
On Windows the thing is more problematic. Allocating the max avail memory there is hard to achieve at all and if then with some very dirty techniques. That said: What works on system A may fail on B and vise versa.
Overall realistically 374/375 can be done. I did benchmark my miner on 376 on my system, but that might fail on any other. Also here: use recent releases!
Now the thing with reduced mining speed:
There were recent releases of miners that allow to continue mining on 4G card at slowing down speed. See here:
https://medium.com/p/ea8f6298f813Given that technique the speed will go down over time, but actually the card will not really stop to mine until you stop it (or your OC settings stop you, because this modes need a bit more juice / relaxed uv). But that way you are able to go on some epochs until you find it unprofitable.
Here an example for a normally 30mh/s card using this:
So well ... making long story short: It depends!