How to charge a 4S 18650 (16.8V) battery pack with a 5V USB input source?
When one of your suggestions is &quot;bypass the BMS&quot; that is a bad idea. The point of the BMS is safety. Safety matters. If you are going to break your pack up into parallel cells, you want to completely isolate them from each other. That is hard to do well. So my advice is to not do it that way.
You other option, option A leads to some more questions:
How are you asking for charging current out of the USB? The USB spec says you only need to provide 100mA (or may be 10mA if you want it to be fully backward compatible) at 4.5V for USB. Making 450mW, So at your charging voltage (4 cells, 4.5V each, 18V) that makes a current of 25mA into your pack. Even if you draw the max official standard USB current (0.5A) that only lets you have: 2.25W, at 18V that gets you... 0.125A. This pack is going to take a long time to charge up, even if you have managed to make a fantastic and impossible 100% efficient boost converter. More realistic, 85% efficient, 4.5V, 0.5A (2.25W) in, 1.9W out, 100mA into your 2000mAh cells... 20 hours to charge up the pack.
Yes there are way to get more power out of power supply designed USB sockets. Also it is possible to get USB adaptors which push out more current. But if you're doing that, why not get a proper mains adaptor? They can be bought pretty cheaply. It depends on what this pack is for, but if it's a mass market thing and you are using USB because it's everywhere, your charge times are going to be really long. Get a proper supply. If it's just for home use and you can cope with a 20hr charge time, and can design a decent constant current boost converter, then that is a possible route, but I'd advise against going that way for safety, convenience and cost.
I would advise: get a suitable constant current power supply to charge your pack up straight from mains. And 18V, 1A supply (assuming your BMS has all the required safety features in it) would to a lot better job.