You can't have all four I think, either Jessica and Milly, or Laurel and Lauren. Inviting only Milly, or only Jessica still brings both of them to the tent.
To get Laurel and Lauren, talk to Lauren first, ask her about everything, she should give you a hint when you ask the correct question. Ask her about sex, you will be prompted if you want her to go to your tent later. Say yes.
Go talk to Laurel, tell her about sister, then ask her about sex, possibly two times, ask her about blowjob, and finally same as Lauren, ask her about sex and she will go to your tent later.
You can't have only one of them, you must invite both to the tent, or else neither will come.
Getting Jessica is the easiest, ask her about everything, she should give you hints soon.
Then ask her about sex, after that once again, you can invite her to the tent.
You need to talk to Milly about everything sexual, couple of times, try kissing her, all that stuff. I don't really know what triggers her, you just need to ask her a lot of times. But I think it's ask her about sex. Get her to go to your tent later.
As far as I know, you can only have sex with Laurel and Lauren together, or Jessica and Milly, if you choose three women, only two enter, and send the third one away. Laurel and Lauren will leave if you invite Jessica and Milly.
Correction: Inviting only Jessica, or only Milly, still brings both of them to the tent, so you only need to invite one of them.