I have a S150 and a Forte model 6, I have not heard a model 4 but I know they have some very good reviews and are supposed to be a very very good amp. It was that reason why I bought model 6 and it is a very good amp. I got both amps within a month or so of esch other and have had not a lot of time to try them in my system, for the past 6 months due to health problems with my brother I have been at his place most of the time. I live in Vancouver and my brother lives in the Mountains the other side of Whislter B.C. No A/C there we run off a generator and no computer. I say this because I owe a few in this thread an email, sorry Jeff but I'm home for the winter now and Ar_t I owe a big email to and a big THANK YOU for telling me about how to Modifly my S150 by moving Transformer away from the circut board. The S150 has the Transformer bolted to the back of the faceplate and is butting up against the circut boards more the left ch. and I guess the magnetic field of the Transformer reacts with the circut boards. I moved the faceplate with the transformer a full 4" forward from the amp (circut board). The way I did it is one can put it back to stock without anyone being able to tell. But will I NO,NO,NO,NO, The S150 was Know as Thresholds lower powered amp in the S series but many thought it was the best sounding and as you can hear with your own ears it is a sweet sounding amp. OK,OK, you want to know how it sounds. Well it's like you went out and spent a lot more $$$$ on another Amp. The differance blew me away I didn't think the mod would have that much affect, was I wrong, the sound stage was much wider and grew in height and depth, placement of the instruments seemed better with more focus and the Bloom of the instruments is what really got me I was not prepared for that, it took a little getting used to there was so much more bloom. I run a pair of Maggies which are 4 ohm and my plan is to run the S150 on the top end + mid and the Forte 6 on the bottom of my Maggie III but my IIIs are down and over the winter I will do a rebuilt so right now the S150 is on my Maggies MG1 imp. I guess you know which amp I would pick, the S150 a very good amp made even better Thanks to Ar_t. I have on order some Vampire Pure copper female RCAs and just got some Eidson pure copper binding posts and a furtech IEC incert and when I was checking the fuses a good size piece of metal came off the inside of the fuse holder so I ordered some cryo jena labs fuse holders Although I might by pass the speaker fuses (output) and live dangerously. If you keep the S150 and want more info email me and I can help you with the Mod. Ofcourse you can talk to Ar_t who made this mod possible and I would just again like to Thank You Ar_t for helping me with this Mod. Oh one more thing can anyone tell me if the other S series amps have the Transformer on the back of the faceplate. Thanks Ken.