Because you mentioned you're trying to learn and improve your English I thought I would try to help a little:

my favorite race, i wish i could raise one of those since im 4 years old

An example of a better way to write this in English might be

"That's my favorite breed. I've wanted to raise one of them since I was four years old."

"im" or "I'm" is short for "I am" and is used to say how you are right now.

"since" has at least two meanings in English:

1) It means "because"

2) It means that something is still true and has been true for all the time after some event or some older point in time. (people can say the words "ever since" and this makes it clear that they mean #2 not #1 when they are talking about something that started a long time ago)

So "since im 4 years old" can be misunderstood as "because I am four years old right now."

So people reading "i wish i could raise one of those since im 4 years old" might think that you are saying that you are four years old right now. They would think that you are saying that because you are four years old right now, you currently wish to raise one.

English is a difficult language to learn, but you are doing good! Keep practicing! I wish I knew Portuguese so that I could help explain this more clearly!