I don't know if it would be possible throughout the day as
there are also freight movements on the line. Presumably
a fifth passenger train would mean a regular service of a train
every twelve minutes and the twelve minute gap might not
be sufficient for freight trains. It's noticeable they already
follow closely on the heels of passenger trains.

The other issue that will arise if patronage grows is Platform 2 at
Blackhorse Road Station. A very large proportion of the passengers
east of Blackhorse Road change there to and from the Victoria Line.
In the late afternoon/early evening the narrow platform 2 is quite
crowded and some passengers wait on the stairs or the footbridge.
TfL will have to widen Platform 2 at some stage. Land is available!

I hope they also sort out the public address system on that platform
which distorts the messages.