Great work on getting to step 3! Shawn is right, in that as you play more you’re lung capacity will increase, and it’s a good idea to keep a firm diaphragm as you have more control that way.
Having said that for now you can try speeding up the tune, so as to use less breath. Or you can also hold one note in place of those four until your more comfortable.
Also, just make sure your not breathing in through your nose as well as your mouth! That can be a factor in this for some.
Finally, stay relaxed and keep on playing. It will come!