to you. I know how worrying it can be to spot during pregnancy, but please know that it is very common. Some women all out bleed and still have healthy pregnancies. Of course, bleeding, especially accompanied by cramping can be the beginning of a m/c, but if it is, there isn't anything you can do except wait and see how things proceed. Also brown spotting indicates old blood, which means this probably happened a few days ago. You mentioned that your cramping was about a week ago? Usually a m/c will go from cramping/spotting to all out bleeding within days. You said you're 4w3d today? So you're about a week past the time of your expected AF right? A m/c from late implantation/short LP (chemical pregnancy) would occur around the time of expected AF, not a week later. Implantation spotting (which would occur between 6 and 12 DPO) is quite uncommon and it is unlikely that what you experienced just prior to AF was from implantation. It is much more likely that it was from hormonal fluctuations around the time of your expected AF and your body adjusting to pregnancy. I spotted at two different times with my first pregnancy and everything was just fine; I hope the same is true for you!
If you're really worried you could request beta hCG tests from your care provider. It can be comforting to see nice doubling numbers. Sending positive thoughts your way!