How can I split a 5v power supply into 4 isolated (no shared ground or live) channels?
While it is possible to charge battery cells connected in series individually by using isolated power supplies and chargers (or by using one supply and one charger and switching them between cells) it is not a good idea.

The engineers trying hard to ensure battery remains balanced, hence all those additional harnesses for measuring individual voltages and battery balancing devices.

Even small discrepancies (due to manufacturing process or component precision) in multiple chargers will lead to unbalanced battery. Using single charger switched between cells is much better in this regard but it will increase charging time, of course. And it creates a possibility for accidentally skipping some cell with drastic results.

There are really good charges for up to 6 cells that will not break your bank, like iMAX B6 and will make the whole process much safer. I'd suggest investing in one.

But if you really inclined on doing this your way, already suggested option of using SMPS (or even good old transformer) with multiple outputs is certainly plausible. It might cost you even more than the charger, though.