I do not know the answer but I was doing some math while thinking about your question (good question by the way!)....

PW can grow to a height of 6 to 10 feet in ten years. It varies based on many variables, such as climate, length of your growing zone, etc.

Four feet would then be around half to 2/3 that 10 year high growth... but achieved in just one year? Hmmm.

My brain melted but Wow! if it can do that. It is such a nice large shrub with large panicles and leaves!

I too would like to hear if anyone has pruned PW to the ground and then measured how high it got in one year. Ostrich had a PW before he moved but I do not know if he ever pruned it hard. Unprofessional also has/had one too. You could try to send them a Gardenweb email if you wish.

Just wondering, Layddi, why do you ask? Are the three PWs planted too close or something like that?

Luis..Thank you for taking the time to respond. I am new to this forum and find it very informative. I am not sure who Ostrich or Unprofessional is but hopefully he/she will read this and let me know. How do you send them a Gardenweb email?
I just had a very large landscape job done and purchased limelights and pinky winkys. I just assumed that you could keep them at whatever height/shape you like, like you can do with rhodo's, azealeas etc. I have planted 1 pw and two limelights in a semicircle (with the PW in the middle of them)on the front left side of my house by the driveway. In the middle of them I planted a red bud tree. I want the tree to be the focal point and not have 6-10 ft PW and limelights around it. That is why I would like to keep them around 4 feet at most. I have them in a semi circle at the end of my garden bed, then to the right I have sedums and knock out roses which stay fairly small, so not sure if the PW and LL will be to overpowering with the rest of the bed. From what I have read, it seems easier to control the PW than the LL so I thought I could swap the two LL for two PW I have also planted on the side of my house...your opinion would be much appreciated.....thanks so much!

If you are going to be swapping plants, why not get all Little Limes. Little Lime is a shorter version of Limelight. It should stay closer to your desired height and therefore won't need such extreme pruning.

Kate