On ebay I bought these BeoLab 4500:

 but from the pictures I realized that were made in 1988. On your site it says that the BeoLab 4500 go into production by 1991. Is it a problem? are these beolab 4500 original?

The year you see on the type sticker is just the copyright year for the design, it doesn't tell when the speakers were manufactured.

Type 6721 is definitely a Beolab 4500. According to the serial number, this pair should be from 1994, middle of the production run

That's a good price if they are in good order! (edit: let's say an OK price - I just saw the dents)

I'd say these are from 1992 or 1993. I happen to own a similar pair myself, known to be from 1994 and with serial numbers starting on "10". Mine also have SW 1.1. I think the difference between the software versions may be minor, possibly just the addition of the CDV source.

There are several slightly different cheat sheets for serial numbers in circulation, but this is the one I believe is correct:

 1****** - 1984
 2****** - 1985
 3****** - 1986
04****** - 1987
05****** - 1988
06****** - 1989
07****** - 1990
08****** - 1991
09****** - 1992/1993
10****** - 1994/1995
11****** - 1996
12****** - 1997
13****** - 1998/1999
14****** - 2000
15****** - 2001
16****** - 2002
17****** - 2003/2004
18****** - 2005
19****** - 2006
20****** - 2007
21****** - 2008/2009
22****** - 2010

The system seems to have been introduced in 1984.

Thanks psand, I bookmarked that.

I've been using this old thread which doesn't go that far back.