Production planning problem(dynamic programming):

I have a question (figure attached) about Mar month, no.5:

- there is 3 on the arrow between (5.0) and (4.1), how we obtined this 3? and there is 4 on the arrow between (5.0) and (4.2), how we obtined this 4?

- Second: why in Mar I have (5.0),(5.1),(5.2),(5.3) and start from 0 where in Apr I have just (4.1),(4.2),(4.3) and start from 1?

Any one can clarify to me in details?

Regards,

2 0 0 0