That is definitely not decided yet... but if you look at the other platforms, they usually have a 7-8 year lifespan after introduced in the pricelist. Eg. SRX3400 introduced in 2009 and went end of sale in June 2017.
Remember that the platforms will still be supported 5 years after end of sale.
SRX4600 was introduced in the pricelist back in November 2017 so I would expect it to have many years ahead before being announced end of sale in favour of newer platforms.