Need some advice

I am wiring together 4 6s 4500ah lipo bricks to make a 100volt pack and then paralleling 3 other packs just like the first one for a total pack of 100volts at 18ah. My question is I want to hook my new chargery bms up should I put my battery's in parallel first and then hook them up in series or can I keep in series groups and then parallel them.

If you never intend on taking the pack apart, the best way to do it is to make lots of 1S4P packs, then join them into 24S configuration. The reasoning being that parallel packs soldered together will get less unbalanced than parallel packs joined by balancing wires, and you will get less voltage sag from additional connectors and wire. It is fairly minimal at low currents though (which I assume you're using since you have a high voltage, low capacity pack).

If you do not want to to go to that effort, then it doesn't matter which order you do first, as long as there is no load on the pack until the battery is completely assembled.