I respectfully disagree. One of the most common reasons given that promote abortion is that its safe, safer than having the full-term birth. If an abortion increases the risk of cancer by 4%, then these ideas are no longer valid.
Ummmmmmmmm, no.
Those ideas are only not valid if abortion increases the risk of DYING of (not just getting) breast cancer by a higher % than having an abortion instead of carrying a baby to term and having it increases the risk of death during childbirth.
Childbirth isn't nearly as risky as it was for most of human history, when death was routine (tens of millions of women have died in childbirth) but there is still risk involved.
Pretty obviously, a 4% increase in breast cancer (and only a fraction of that 4% increase in cancers are increased deaths) is not going to be more than the increased risk of death by having a full-term baby or c-section over the risk of an abortion.
If you are opposed to abortion the whole breast cancer thing is a stupid argument and a waste of time.