Awww! I am sorry! It is hard to say what caused their demise. Generally goats are finicky creatures that require different dietary/mineral inputs than most other domestic livestock. Without a detailed history it's impossible for me to speculate about why that happened. Birthing 4 kids is alot of work for a doe. Any others possibly bred watch for full udder, usually right before kidding (12-24hrs)their teats will strut out tight an full. (but not always) Their girly bits behind will get long and loose and may string some mucus. They may paw the ground, stretch alot, rub along the fence lines more than usual, stand around curling their upper lip. Some want to go off by themselves, others want to stay with the herd. Keeping them out of freezing weather and rain is a must if you think kidding is eminent. Pat Colby's, 'Natural Goat Keeping' is a book I have learned a lot from and use it whenever a problem comes up. Some goats... Or members within a breed, such as boers, have been bred for maximum income and not so much hardiness and mothering ability. I have read of people buying nice papered, bred does for their kids 4-H projects and them being terrible at birthing and mothering.... Again, I am very sorry you lost those kids...
Edit to add... For her I would watch her udder. If your not going to milk her then I would just keep an eye on how full it is, how long it takes to start reabsorbing the milk. It should start to shrink in 1-3 days. If it remains tight and full for more than a couple days she may get mastitis. Generally they act sick, go off feed, have a fever when they get mastitis. If she were mine I would also not feed much commercial feed ration for a bit, but all the high quality hay/browse she wants. That'll help stop milk production.