These sentences represent different verb tenses and conditional forms, each conveying a different sense of time and condition. (B1 to C2 levels)

1. “If I have money, I come.” – This is in the present simple tense and expresses a habitual or regular action. It suggests that whenever you have money, you come, as if it’s a routine.
2. “If I have money, I will come.” – This is in the future simple tense and implies an intention or willingness to come in the future if you have money.
3. “If I had money, I would come.” – This is in the present unreal conditional tense. It suggests a hypothetical situation in the present where you currently don’t have money, and as a result, you wouldn’t come.
4. “If I had had money, I would have come.” – This is in the past unreal conditional perfect tense. It refers to a hypothetical situation in the past where you didn’t have money, and as a result, you couldn’t come. It emphasizes the completed nature of the action (not coming) in relation to the past.

So, the difference lies in the verb tenses and the temporal contexts these sentences refer to.