Regarding the 4ths to 5ths tuning for a four string bass and as to whether there is any more tension requiring the bass bar to be altered, here's something to consider: On the TI website, the factory specs for tension for a set of S42 Weichs is just shy of @ 260 lbs total tension. The 5ths Mitts set is right on 260 lbs, or just a fraction more, total tension for the set.
So it would seem quite the contrary: a well-designed 4-string set of 5ths tuning strings should be about the same as a conventional 4ths set of strings, and because everything is going down, a 4ths set of strings loosened up to the 5ths tuning will actually have less tension.
This is, of course, a completely different issue from adding a 5th string, which if a player likes the feel of a certain 4-string set, and adds a matching 5th string, by definition, that's adding roughly 25% more tension to the top of the bass.
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