My third baby didn't have colic but she just absolutely refused to sleep on her back, pretty much from day one. My other two I always put 'back to sleep" but she just would not tolerate it. I finally gave up and just let her sleep that way.
Growth spurts go around 2 weeks, 4 weeks, and then a big one at 6 weeks, then smaller ones at 3 months and 6 months. Sounds like fairly common behavior for a 4 week growth spurt. Sometimes fussy is caused by over consumption of the foremilk, too.
I am aware that the official age for cereal...