All 4 would only be needed in time for Hi Band transmissions (quite a long time off still), until then 2 would do.
That should give the full picture now, I'll end my side with a bit of further detail.
Originally Posted by Geoff D
It depends on what you want to do as far as connecting up decoders.
As the universal ports aren't needed with the 3 x Exploras, the SLNB is fine as an option. Just the revealed extra cost factor then.
I think Geoff is mainly thinking of the heartbeat (HB) cables, with the SLNB one can alternatively have the HB simply work via the SLNB cables instead. That choice is influenced by any RF distribution factors and is less likely to offer any advantages after having already set up unless you want to reclaim cabling space in any cabling conduits in use. Maybe it could be helpful with some of your old f/t usage.