If all 4 were on hard nodes you’ll need 3 months considering refreshing each node once a day, because you can farm them at the same time. (So about 4 months because the last character should be available 3-4 weeks after the first).
If all 4 are on cantina nodes you need 1 month for each considering 3x100 refreshes, so 4 months because you need to finish one to start the other.
Shards on hard mode: you can farm relic data with cantina energy, great. Shards on cantina node: you can farm kyros with regular energy, great.
As long as you have enough crystals to buy 3x100 energy a day, it doesn’t really matter on the long run.
However, if you have a young account with limited crystal revenue and can’t buy those refreshes, Starkiller City is a long drive and you’re better off going for someone else, like a GL, until the prerequisites are on accelerated farm.