To expand a bit on the answer from Ed Beal, each 18v battery pack includes a BMS circuit which keeps the voltages of individual cells more or less the same. This helps prevent the identified issue with good cells getting over charged. When connecting multiple packs in parallel, there would be no such BMS to monitor and correct for differences between packs. In the end, you'd end up having to periodically charge individual packs no matter what. All that said, if you used all new packs with identical cells inside, they would probably stay pretty well matched for some time.
However, it still may not work with a standard Ryobi charger because we don't really know the charge algorithm. First of all, it detects what kind of battery (nicad vs lithium) is plugged into it. I don't know how it does that, but I can easily imagine there is a handshake between the charger and battery. That handshake almost certainly would not work with a bunch of packs connected in parallel. Moreover, chargers often have failsafe timers. If the pack is charging for a certain time, but is still not full, the charger stops out of an abundance of caution. So, it might not be a plug it in and forget it proposition because 4 packs in parallel will take 4 times longer to charge.
Last, I don't know what the application is or your price sensitivity, but you might consider an off-the-shelf ebike battery. These are generally 36V, but it's easy to step that down to 18. Ebike battery chargers are pretty cheap and indeed, are usually bundled with the battery. Likewise, these batteries include builtin BMS circuits to keep the cells balanced and prevent over/under charging. See ebikes.ca for some examples.