The .404 clearly falls in between the two rifles that you already have with standard 400 grain bullets. The distinction is blurred further if you load woodleigh 350gr loads in .375 bringing it close to the 400gr .404 or if you load 450gr woodleighs in the .404 it moves closer to the 500gr .458 Lott. Having said that, the 550gr woodleighs move the .458 Lott into .505 territory...

Another way of looking at it depends what you plan to hunt. A .404 hits a buffalo much harder than a .375. However, Ian Nyschens was of the opinion that a .404 was not heavy enough for elephant in thick bush. He might have changed his mind if the 450gr woodleighs were available though, given that he opted for a .450NE.

If elephant are not on your list, a .404 with its .423” bullets in either 400gr or better 450gr loads should offer all you could ever want in a buffalo rifle. But then that could make your Lott redundant! On the other hand, if you wanted to go for the 505, your Lott would also be redundant, so you have a real dilemma! A nice position to be in! Given the rifles you already have and the ones you are considering, you are never going to be in the position of not having a suitable rifle for whatever you need to hunt.

Seriously, you can’t go wrong with either, even considering what you already have. So I agree and disagree with Dave

If it was me, I’d keep the .375, get the .404 and lose the Lott. I’d then hold out for DR in either .500/.450 regulated with 550gr woodleighs or a .500NE