I would say it could be a CHEMICAL Preg, google it and look into it, i just had one. I got a Pos preg urine read 17 days after i Misscarried, when a Mc is this early it is considered a Chemical preg, meaning you can not see anthing on the sono only chemicaly, meaning urine or blood sometimes what happ is you get a pos read and then it fades as you lose the baby, i believe with a Chemical preg, the baby is not able to implant it self into the lining of the uterus, but you get the a post read. Google it and see if it matches up what you are saying.
1). Too early to see anything on sonogram. Levels need to be 1000 or higher in order to see sac. So in 4 days you probably still won't see anything. I'd wait 2 weeks.
2). It could be a chemical pregnancy.
3). Doctor's urine tests are not as sensitive as the ones you can buy.
4). Bleeding during pregnancy doesn't always indicate a problem but may support the theory she is having a miscarriage.
I wish you both much luck!