Don't really know the going rate but it seems reasonable.
FIT has ended. If your install is MCS certified (signed off, MCS certified panels inverter, etc) then your energy supplier should still pay you for export - not much, a few pence/unit i think. If your install is not MCS certified then they don't have to pay you anything for exported energy.
If you're going down the battery route then you wont be exporting much. If you fit a diversion controller (or if the fitted system incorporates that function) to divert any excess production to immersion/storage heaters you won't really export anything, except on sunny midsummer days.
Bear in mind that a battery isn't 100% efficient - you won't get as much out as you put in, and in midwinter the standing loss of the battery system may absorb most/all of what you do generate from 4kw of PV on a dull day.