If he really decides to do it, he can do it. This is OK. As for 3 and 4 you seem to refer to an unfulfilled action and hence it is - If you had decided to dot it.
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navitasan1-If he really decides to do it, he can do it.2-If he really decided to do it, he could do it.3-If he really decides to do it, he can.4-If he really decided to do it, he could.Grammatically, they are all fine. 1 and 3 are Conditional I; 2 and 4 are Conditional II.