Try not to worry too much at this stage lah, a low implantation at 4 weeks is actually quite common and many pregnancies go on to be perfectly fine as the placenta migrates upwards naturally as your uterus grows. Your gynae is right that it really depends on where it ends up by the second and third trimester, so that 2-week follow-up will give you a much clearer picture. At 4 weeks you also wouldn't see a heartbeat yet so that's completely normal, so just take it one scan at a time and try to rest easy for now. I know the googling spiral is real, but honestly most low implantations sort themselves out, so just keep that in mind when the worry creeps in.