I am fine about a challenge / discussion about why a card should be in the SB or in the maindeck, thing is that your approach is very black / white.
Yes I / we might board in the Bridge alot. That for me does not mean that it therefore warrants a maindeck slot.
I will take this the other way around, you say the 4th bridge has to go maindeck, so what has to go then?
If I look at the maindeck, I really like the manabase atm so that would be a no go area for me, so;
Manabase:
11 Mountain = no go
4 City of Traitors = no go
4 Ancient Tomb = no go
4 Chrome Mox = no go
4 Simian Spirit guide = no go
This is the core of the deck, if this starts to fall apart, it is time to change decks.
Prison Package:
4 Blood Moon = no go
3 Magus of the Moon = no go
3 Trinisphere = no go
4 Chalice of the Void = no go
3 Ensnaring Bridge = no go as that would be swap bridges :)
Wincons: (If I could I would even go up 1, but not a Pyromaster)
4 Chandra, Torch of Defiance = no go
2 Chandra, Pyromaster = no go
Various slots:
3 Fiery Confluence = no go, this is the "Allstar" in the deck this the nr 1 Wishtarget in the board.
4 Burning Wish (The versatility is the thing that I like here, get LD for their basic, get a sweeper, get artifact removal, get creature removal)
That leaves 1 card;
3 Sorcerous Spyglass
I like
Sorcerous Spyglass, it is versatile, it gives you information, it is not a 3 drop (and for me this is HUGE atm) it gives you something to do next to
Chalice of the Void with just 2 mana available.
So in this version of the deck I would leave the 4th Bridge in the board.
This is the way I see it now, and I tried to explain the reasoning behind it.
Jelmer