The Ketron X4 and SD40 have the same basic sound engine, and I feel that even today, it's excellent. I also own a Roland BK-7m, the most advanced arranger on the market today, and in certain sounds, the Ketron excels even better than the BK. The SD40 is the later version, and likely has some nice improvements over the X4.
In terms of sound quality, I'd match it up against the best on the market, it's pretty good.
That said, I'd never suggest anyone make a purchase based on some words you read on an internet forum. Go use it with your accordion see if you like the styles It offers and how it sounds to YOUR ears.