4 month old eating habits ?

my daughter is a month or two ahead of her milestones . her pediatrition recommended i start her on solids bc she is ready already . we had her do apple sauce that day ( monday ) .. the dr said change foods every 3 days to watch for allergies . - she did greenbeans today , she ate a whole cup or gerber 1st stage mixed with 2 tbs of rice . she opened her mouth & leaned to the spoon and everything . only being her 3rd day on solids . is that normal ? do most babies eat that much the 3rd day ? do they pick up on leaning & opening her mouth to eat so quicky ?