I found an online activity points calculator. For walking for 45 mins it says it's 4 points. Does that mean I get four more points for the day?

Short answer: Yes. If you rack up activity points each day, you can look at them as "extra" points you can use as you see fit. Some people use them as extra points to allow them a little something extra each day; others (like me) don't use them at all.

According officially to the plan: Yes.

They're your points and you can use them as you want/need.

I personally do not eat activity points. I do, however, use the weekly points.

I too do not use activity points. I just use daily and weekly points. I save the activity points for added weight loss and/or if I happen to go over my weekly unknowingly by miscalculating during the week - that way they are there more as a cushion to fix anything I may mess up. I like that added security.