Stopping development of MT4 does not mean MT4 will disappear. Perhaps not new builds will be released, but not stopping the most widespread trading terminal in world.
MT4 will stay for many years forward.
They are still releasing updates
but they also increased fees for brokers for mt4
to push them to mt5
I believe this would be long process and the MT4 will work for many years. The switch from MT4 to MT5 will require a lot of work for the brokers itself and changes in contracts, accounts, compliances and etc.
Something I have heart and not sure if it is true or not, but: 'Metaquotes have left a port in MT5 constantly open and they could have access to your terminal' - This is what I have heart and repeat - not sure whether it is correct :)