Hey there, @twotimothys!
Thank you for your questions. If you daughter is already an Existing Boost Customer, we will need to switch her plan to your plan so the linkage can be possible.
Please keep in mind if you are on this promotion: 3 Lines for $100, by adding a fourth line, the plan will go up to $130.
If your daughter is bringing her line to Boost from another carrier, she maybe eligible for the free phone promo, and you will be able to add her line to your family group, the bill will go up to $130.
We recommend you to visit the nearest store in your area to get any additional information on the Boost Dealer promotions.
If you are referring now to the Family Plan Promotion (4 for $120), it is possible to get this promotion for all 4 lines you want to link in your family group. Please be aware that it is a limited time promo, that will end on 10/24/19. Always, if your daughter line is from a non Sprint carrier, please visit the nearest Boost store in your neighborhood to take advantage of the offers. Thanks.