Topically at this age they only need 30ml so 70ml is definitely enough. If it's painful turn down the strength of the pump it shouldn't hurt. I have exclusively pumped for nearly a year since my daughter was 3 days old (she's 1 in a week) lanolin on your nipples before pumping and check the flange is the right size, if its too big it will hurt
@Helen great tips but just an fyi for OP, lanolin can be a huge irritant for some so maybe try a small amount first to check you aren’t allergic or find it irritating!
First step is measure your nipples to ensure your pump fits you as that could be causing the pain
Colostrum is the first milk. Whatever baby has had will be colostrum but might also be a mix of your usual milk now on day 4 as it’s normal for it to take till day 4/5 for the milk to come in. Have you put baby to the breast at all since birth? If it’s painful it might be that the pump doesn’t fit your nipple properly. Have you got any local feeding groups you could attend for face to face help?