When will London telephone numbers starting with a 4 come into use?

From 2001 onward London numbers changed to area code 020 with a 'director' code starting with a 7 or 8. After those ran out (they started issuing number starting with a 3 (i.e. 020 3xxx xxxx).





Presumably at some point those will run out and the will have to start on the 4xxx numbers. Any idea when that will be at current rate of issue?When will London telephone numbers starting with a 4 come into use?
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