I have seen a few Tracker pontoons with a stabilizer log in the center. It looks to be about 10 feet long, maybe a little shorter, and is set back in between the outer logs by about 2 feet. I don't think 4 inches deeper on the center log will be felt on the boat at all.
If you are looking to just keep the bow of your boat up I would put it wherever you want to. I put a 14 foot center log on my 20 foot boat and it did a lot to stabilize the boat underway, but did not improve performance. Mine is even with the other two. There is a link to my third log saga on this page somewhere.
2006 Forester 19 Fish (new deck and carpet, Pontoonstuff interior, 2019)
1996 Mercury 50 ELPT4S
1983 Sea Nymph FM171 Striper (complete rebuild from hull up, 2014)
1985 Johnson 70 J70ELCO
Raystown Country, PA