Problem downloading an IBook on my Iphone 5

Just purchased an IBook and it downloaded right away on my IPad, but it won't download on my IPhone. When going into the "purchased" section of IBooks I get a message saying I must have an IOS device with IBooks 1.3.1 or later and IOS 4.3.3 or later. Both my IPad and IPhone have the same operating system. Any suggestions????