Have you tried to benchmark with older DAG Epoch? (60 or 80 for example)
What results are you getting?
(Just to be sure that the benchmark is working correctly)
If you are getting hashrates above the normal (let's say 60Mhs for epoch 60) then this is not normal. Cards never got that high hashrate.
Because from what I have read, the DAG size 4GB limit will be reached on September 2019, but POS will come earlier.
Some answer from an other forum regarding DAG size:
The DAG started at 1 GB at the time of the Frontier launch, and increases by approximately 0.73x per year. That puts the current size at roughly 1.35 GB as of mid-January 2016 (feel free to comment with an exact link if anyone has one).
Following the same crude approximation:
The 2GB limit will be hit around mid-December 2016
The 3GB limit will be hit around mid-April 2018
The 4GB limit will be hit around mid-September 2019
However, it's likely that the switch to PoS will occur long before then.