I use an IFF Ampliator 5x7" enlarger and a 5x7" mixing box with 4x5" negatives. Works very well! I doubt't that the 4x5" mixing box would be significantly brighter.
Thanks at all ! so there are no problems ! ... but i want to take a closer look to the enlarger next sunday , any suggestions on what i should check ? somewhere i read about asbestos condition ...