Well, I have played Avernum 1-3 few times trough and tried BoA, and as I now tried A4 I was, well, disappointed. Not only were the graphics like the Geneforce, but the game engine too. I liked the clickety-click Avernum movement system, this point and click just is not Avernum! And when I made my party I couldn't find my favorite characters I had played in the A1-3, what a misery.
Geneforce is good game at itself, but mix of Avernum and Geneforce... oh no no! I probably play A4 just to see how story continues, but new interface and graphics sure make it painful.
I hope that if there ever will be A5 Spiderweb changes back to good old Avernum engine. Well, they're probably hardly going to use quaint Avernum engine. The masses want something new! Heck with all those old-fashioned people who can't change their habits and fondnesses!