It's great that your boys are enjoying soccer. Having an older brother will be a great influence on your 4 year old; it certainly helped me with sports in general.
At 4 years old, your son is still in a learning by imitation as well as exploration. It is not surprising that he does not really take charge yet on the field, even if all players were the same age.
I personally believe that 4 and 5 years old is a little young to expect much competitiveness in an organized game such as soccer. Children at this age are still learning so much about their environment and especially family structure (which you described as he watches and plays with you).
I recommend that you allow him to explore soccer in the same way that he is exploring everything else around him. He will continue to imitate you and your older son. Provide him with the opportunity to play with you and his age 4 peer group, but don't expect him to begin to understand the team concept of soccer yet.
It is more important for him to socialize with his peers and feel comfortable in the group setting. Your son will entering a selfish phase soon enough, don't worry, he'll attack the ball plenty when that happens:)
Anyway, both you and your older son have an immeasurable impact on your 4 year old. Playing with you and watching you play will give him all the encouragement that he needs. Give him time, be supportive, but never push where he doesn't want to go in terms of sports.
Remember, there is no standard of development for kids, each matures at his own pace.