Hmmm....it could be that your routine needs a bit of a tweak perhaps? Angel/textbook babies usually take to the average A times very well ... and for 6 months, the average is between 2.25 and 2.5 hours. So your routine might look a little like this:
4-hour E.A.S.Y.
E: 7:00 wake up and feed
A: 7:30
S: 9:30 (for 1.5 hour nap)
E: 11:00
A: 11:30
S: 1:30 (1.5 hours)
E: 3:00
A: 3:30
S: 5:00 or 5:30 or somewhere in between: catnap [most babies drop this between 6-8 months, but there are many variations] for 20-30 mins
E: 6:00
A: bath/wind down and milk before bed
S: 7:30/8.00 bedtime
E: 11:00 dream feed (until 7 or 8 months, or whenever solid food is firmly established)
What do you think? I would only use PU/PD as a last resort or if there is a sleep prop issue to be resolved.