There are two iphone4s between my friend and me. She communicates with me, but she can't send imessage. He can do it with others, I can do it with others, but we can't.

Hello, you can open iMessage in Settings → Information and use your mobile phone number Apple ID to receive it. When sending a short message, if the contact you choose has activated iMessage, iPhone will automatically use iMessage to send it. There is no short message fee for sending iMessage information, which requires effective wireless network transmission and reception.