The Sonett cards are going to cost you in excess of $600 now be warned. The Amiga tax has now been applied to it and people know they sort of work. For the money, it would cost you to beef up that 4000 I would wait and see what the Amiga 1222 due out by the end of next month is like.
That being said... I would get a new one from Elbox or AMigastore.eu. Amigakit also has them from time to time in stock. That way you get better support. They are a pain to get stable and everything installed. The hardware you use is very specific and can be extremely picky at getting configured so read up on it all before you make a purchase!
Some A4000D models just refuse to work well with the mediator as well. I know several that got endless crashes and yellow screens with them installed. I use one in my 4000T and love it though, zero issues for several years.
