In your instance I would get the shot rather than worry for the remaining 4 wks if things were okay.
I am (-), my husband is (+). I got the shot at 28 wks during both of my first two pregnancies, after as well for my 2nd son because his type was (+), and I declined it for my third. My OB(who just happened to be a very natural friendly, high-risk maternal/fetal medicine specialist) for the third said (and I looked it up, it is true) the chance of blood mixing in the right time frame from the shot was very minimal, and that many countries in Europe don't even do the routine shot during pregnancy, only after if the baby is positive.
That OB moved, and for my fourth, I had a different OB who was not at all comfortable with my declining the 28 wk shot, but after what my prior doctor had said, with more experience, I did not want it and refused. When my youngest was born, there had been a small undiagnosed abruption and for several weeks I had been bleeding out into the amniotic fluid. Thankfully, she was rh(-), but it was a scary few hours until that was determined after her birth. If she had been (+) wecould very well have been one of those rare statistics. I am on the fence about it this time, I would like to try to consult my former doctor as he was more educated for his opinion. I'm trying to understand the speculation about how long it is effective after 28 wks, and speculation about how injecting the antibodies could actually harm the fetus in the event of a blood transfer-and in that case, if I'm not planning on having more children, I'm unsure as to how important it would be to get it.
I'm also in an odd subgroup that I have had 3 (-) children and 1 (+) child, because it would be much more likely for them to all be (+) being that it can be a dominant trait (or so I was told by the neonatologist at my DD's birth).