I don't use buses (including the P4) all that regularly any longer but do find checking the live bus tracker just as I leave for the bus stop really handy in determining what route to take or whether to delay the journey.

It's always worth complaining to TFL so that the service delay is logged against that operator - particularly if the bus maintenance is another concern. There are roadworks at Lewisham currently and in Gallery Road (not sure if the Loughborough Junction are ongoing too) which likely doesn't help the P4's timekeeping.

Traffic in Brixton seemed quite jammed when I cycled through at 7.30ish so that might not have helped either.