Can someone please help me. i currently use s26...

Can someone please help me. i currently use s26 for my 4 month old baby and have done since day one. I find now even through she feeds till she's full she's not sleeping enough like 20 min naps all day long. I have started giving Rice Cereal to top up with and sleeping 2 hours, should i continue this every meal to get her to sleep or should i try a different formula, somthing thicker? She is so over tiered i don't know what to do.