Super simple: Convention for connecting batteries in series to load
'Connect positive first' is a convention taught when jump-starting cars, because it's safer than the alternative. If you first connect the two negatives, then have made one positive connection, then accidentally brush the final clip against almost anything, it's likely to be chassis, and will short one of the two batteries. If you first connect the two positives, then any accidental connection with the negative lead will be to chassis, which is where it was going to be connected anyway, so no problem.
If the four 12 V batteries are identical, you can treat them as a single 48 V battery, once they are connected in series. Lead acid can be safely overcharged under defined circumstances (current below a threshold, topping up wet cells as required), which balances the batteries. There's no need for an explicit BMS as used with lithium cells, which must not be overcharged under any circumstances.
There is no need to connect a particular terminal first or last, unless the configuration of your conductors, like in the car example, gives you a specific reason.
Always include a suitable fuse in the circuit when using lead acid batteries. They are capable of very high currents. Thin connecting wires could start a fire, or burn you (I've burnt myself, I've burnt other people, I'm finally learning). Thick connecting wires could sustain high power arcs which could start a fire, or damage the batteries. A suitable fuse means one with a current rating not greater than the current rating of the smallest wire in the circuit.