Feeding baby

So my boy is almost 4 months. Lately hes been waking up 2 times a night to eat. He used to sleep through the night for quite some time 8 to 9 hrs. Hes 14 lbs already so I'm thinking my milk just isn't enough for him anymore. Today he eat 4oz bottles 4 times in 12 hrs. I want to start adding some cereal to his night bottle. How should I start and how much. I'm a first time mom so everything is new to me. Also hes been pooping only once a week since 2 months. So I want to make sure I dont get him constipated.

Also I want to be done with breast feeding. I know its early and a lot of people are making me feel Guilty , but I'm just so done with it. I have about 8 gallons of milk saved up so that might last a few weeks till he is 5months anyone else stopped so early?