Differences between Pi Speakers

What are the differences are between the Theater Series 3 Pi and 4 Pi speakers?
Also, I would like to build this next pair of speakers out of plywood. I´ve had enough of veneering for now and I´ve seen a few Pi Speakers made out plywood so I suppose it would be ok right? Do I need to add any material thickness compared to MDF? I plan to use berch plywood like that in the picture. (They sell 18mm, 21mm and here. BIG (!) price differences betweem them!).
/Erik

Follow Ups:

I meant birch plywood (nt) - Erik Johansson 11:46:37 08/11/02 (10)
- Strengths and weaknesses - Wayne Parham 13:01:31 08/11/02 (9)
  - Re: Strengths and weaknesses - Erik Johansson 03:24:48 08/12/02 (4)
    - Re: Strengths and weaknesses - Wayne Parham 03:49:07 08/12/02 (2)
      - Re: Strengths and weaknesses - Erik Johansson 01:26:07 08/13/02 (1)
        
        Re: Strengths and weaknesses - Wayne Parham 01:46:20 08/13/02 (0)
    - I'm stupid! - Erik Johansson 03:43:33 08/12/02 (0)
  - Begging the question, Wayne - Till E. 17:27:37 08/11/02 (3)
    - JBL 2405 and KSN 1038 - Wayne Parham 19:44:57 08/11/02 (2)
      - Re: JBL 2405 and KSN 1038 - Till E. 20:39:04 08/11/02 (1)
        
        Re: JBL 2405 and KSN 1038 - Wayne Parham 21:04:14 08/11/02 (0)