If 4 inches is your non-erect size:
4 inches is slightly large for a non-erect penis, not many guys I know have non-erect penises over the length of 2-3 inches. Still, there is no normal or abnormal size for a penis.
Will it grow (Erection-wise):
All penises grow when a guy is aroused sexually, like reading/listening to/watching erotic material. Or thinking of sexual intercourse. Blood rushes to your penis and it 'grows'. Erection in other words.
Will it grow (General size):
My cousin 'helps' his non=erect penis size grow longer by masturbating more, and being more 'sexually active'. I heard that his penis grew 1/2 inch in a week just because he masturbated more.
Again, there is no normal or abnormal size for a penis and yes it can grow (only if you 'let' it).
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