The 4th Squad is the additional march slot associated with the Super Monthly Pass. It gives you another squad to send out while your other marches are busy. Its building entry also lists Base level 7, but reaching that Base level alone does not supply the pass's extra slot.
How the extra slot works
An extra march lets you run another assignment at the same time. You still need heroes and units for the formation, and a squad already deployed must return before it can take a different assignment.
The pass describes its extra Squad slot separately from its gathering-speed and PvE benefits. The extra slot is the part that increases the number of simultaneous marches.
Upgrading the fourth squad
Like the other Squad buildings, the 4th Squad gains building power and a march-speed bonus through upgrades. Its level does not determine how many other Squad slots are unlocked.
Squad 4 also has its own research branch. Its Load Capacity improvements affect carried resources, while Hero Squad Size affects unit capacity. For the other permanent building projects, see the 2nd Squad and 3rd Squad, whose construction requirements include Base levels 10 and 20.
For the difference between deployment slots, unit capacity and resource load, read Squads Explained.
