I started doing the stage 2 at 5 months quantity wise since she'd take 2 stage 1 jars. I didn't start meat until 6.5 months, but some pedis recommend waiting longer. I stayed away from the ones that included eggs, cheese, and milk, but everything else I introduced every 3 days.
Re: If you started solids at 4 months
I started doing stage 2 right after 6 months. A lot of them aren't much different besides that they're bigger.