(even if discontinued) could it be working fine with 4G/4G+?.
This device is a 4G device, although it only supports 2 LTE bands - B3 1800 MHz and B7 2600 MHz FDD. I wouldn't buy
this device unless your MNO of choice only uses these two bands to provide LTE in your area.
Note: none Mikrotik LTE devices currently support 4G+
, which (seems it's a common consensus of using marketing BS) involves speeds exceeding "basic" 150/50 Mbps ... usually by using Carrier Aggregation (CA) which means simultaneous use of two different LTE cells operating in different frequency bands.
when a device becomes discontinued, can it still receive firmware updates?
Not really sure to what you refer with "firmware". There are two pieces of software running in a typical Routerboard: routerboot which gets run at boot time (with functionality similar to BIOS on PCs) and routeros which is then running all the time (with functionality similar to OS on PCs).
routerboot gets some updates from time to time, mostly immediately after release of new hardware. Which means that discontinued device won't likely see any update after it becomes discontinued.
routeros is updated all the time and support for discontinued devices gets dropped quite some time after devices get discontinued. Mostly with some major ROS version upgrade (last major ROS release is 6.x and came around in 2013. I expect some legacy devices might loose support with ROS 7 ... which is "right around the corner" for the last few years) which drops support for whole architecture family. I've found a presentation from 2013 Moscow MUM covering news in ROS v6 ... and according to that presentation no architecture lost support at that time.