It is certainly not too late to make a police report. You are not failing any kind of "timely manner" category. A lot of women are too distraught after a rape (especially one like this, where it barely even registers to her in her distress that it was a rape) to go immediately to the police, the way they would if they had been beaten and restrained. Even if you had gone straight to the police station that night, the only thing that would have benefited you was that they could have done a rape swab then to see if he had left any DNA, so he would not be able to claim you didn't even have sex. But even his DNA on your body or clothing would still not be much proof that it was an unwanted encounter, except, of course, that they would have been able to see from your demeanor that you were distraught. All the women's hotlines can help you with how to report and from whom you might get assistance (that is, an attorney or legal clinic). I recommend you contact one.
I could see that was the question, for sure. No, you will not get a positive on a home pregnancy test (or even a blood test) after only 3/4 days. You were already pregnant when you were forced into having sex on the dark road. That's the good news! Get tested for all the STDs but don't worry about paternity.
The bad news is, this was a crime, by the guy. It wasn't classic rape where the guy holds you down, but it was certainly date rape and probably does fit the legal definition of rape -- you were in fear he was going to harm you and you gave in only for that reason. He might not know how frightened you were and so he might deny it was anything but consensual, but it was not consensual. In a fair world, you would be able to report this crime to the police and not be accused of somehow leading him on, or just being a party girl who didn't care. If you have the strength, and maybe with the help of someone from a women's services legal clinic, you should file a police report against the guy. It is not your fault. You didn't have any reason to mistrust him. It isn't your fault that you didn't run away, a woman should not be compelled to run down dark roads in order to avoid unwanted sex. Giving in because you were afraid he would harm you -- sometimes women who make this choice are literally chosing to live versus to die. You chose not to be hurt. But it was rape, and rape is a crime, and crimes should be reported even if they are difficult for the DA to prosecute.
This might be a difficult decision for you to make. But as we can see from the news recently, if someone were to have this pattern of behavior and the victims only speak up much later, they are less likely to be believed. Even if the DA decides the situation is not something that can be prosecuted, there will be a record of your report. And you will feel less like a victim if you speak up.
Back to the question of how soon a pregnancy appears in hCG tests -- the egg pops out of the ovary, and it begins to develop. It has a shell, which it sheds between days 5 and 6 after ovulation. Before that, it cannot embed in the uterus, the shell prevents it. And only after it finally embeds in the uterus does it put out the hCG signal that can be picked up by a pregnancy test.
Go to your doctor between October 10th and the 17th and get an ultrasound. If the doc doesn't ordinarily do them that early, explain that you were raped, but you think you were already pregnant, and you would like a measurement of the embryo from an ultrasound to help you be sure that is true. Be sure the doctor (from the ultrasound) gives you an estimated due date, not just a "weeks pregnant" count. ("Weeks pregnant" counts begin on the first day of your last period and are not helpful for determining when conception was. You need a measurement of the embryo from an ultrasound.) You can take the estimated due date home and put it in a conception calculator online, or just count back 266 days on a calendar, and it will help you pinpoint an estimated date of conception. Probably you will find it had been at least a couple of weeks since conception by the night of the 26th, particularly if your cycles are regular.
If you do prosecute, one thing you can ask for from the date rapist is that he pay for a prenatal DNA test from Ravgen or the DDC. The dates don't point to him being the father, but there is no harm in testing and it will keep you from stressing about this during the pregnancy, he owes you at least that much.
Forgot to add I just want to know if it is possible to get a positive result after 3/4 days, thanks