Paper used in laser printers which feed paper short edge should always be long grain (all laser and copier grade paper is long grain). If short grain paper is used in any laser printer then the paper will curl badly and will cause jamming if you are duplexing. The FS-7000/+ and FS-9000 feed paper is fed long edge on and can therefore use A4 short grain.
NB: Many people produce A5 paper by cutting long grain A4 in half. This makes the A5 effectively short grain which will cause problems. When purchasing A5 paper it is important to ensure that it is also long grain.