I had first trimester bleeding and I had my first positive test at 10dpo. So, before 4 weeks. They positive lines kept getting darker until mine looked exactly like this by the end of my 4th or 5th week. You can't really use a hpt as a guide to see if your hcg is rising, but in my case, when my test looked like this, my hcg was over 10,000 as opposed to my first one where it was only 199 (and that was at 4 weeks exactly). Sometimes, some tests (even in the same box) are more sensitive than others or there was more or less dye in the antibody strip to stick to the hcg in your urine.
Doesn't really mean anything different than a lighter line. It just makes you feel real good to see a super dark postive line. Congrats on your pregnancy! I have a pic similar to this with a FRER in my pics.
Its a good sign dont worry, its showing up darker because you have a good amount of hcg in your urine, which will hopefully mean a strong pregnancy for you :)