Gpu cards with 4GB gddr memory will be good forsome time, 5years at least
Nope.
At recent DAG growth rate, the DAG file should exceed 4 GB somewhere in 2019.
DAG files only apply to ETH and it's offshoots like ETC - no other cryptocoin needs anywhere near that much GPU memory available.
Only for ETH, which should be POS only LONG before then and ETC. Lots of other Ethash coins like UBQ, MUSIC and EXP have a Dag file nowhere near 2GB, much less 4GB.