You could use NiMH with a combination charge system; current limited constant voltage to possibly 1.45V per cell plus a trickle charge of 0.05C for a few hours to top them off.
That can be as simple as a voltage regulator plus a resistor, calculated so the current drops to 0.05C (= C / 20, the max current for continuous trickle) before the cells reach maximum voltage.
For a lithium pack, you need a an appropriate charge controller plus an "protection" board to prevent overall over- or under-voltage.
And, for anything with two or more cells in series, a "balance controller".
It's the balance part that needs the multi-pin connection, but you can easily get balance controller boards to build in to battery packs.
Lack of a balance controller somewhere is one of the common causes of cells bursting or catching fire, as a slight capacity difference can mean one cell overcharges before the overall pack voltage reaches full charge level...
Examples
Protection board: **broken link removed**
Balancing board: **broken link removed**
The protection boards sometimes have balance circuitry as well, but that may be rated at far too low current to be useful.
My rule-of-thumb is the balance circuit should be rated at least 10% of the max charge current and preferably higher.
That may be excess, but for a trivial price I'd rather avoid burst cells...
With those in the battery casing, you can then treat it as a two-terminal pack and use a separate two cell charger without extra balance connections.