Please tell me what it means. I really want to see & use my Pi as an FM transmitter.
Make sure you have correctly identified GPIO pin 4. It's the 4th pin down (4th row) on the inside column.
If you system is heavily loaded, then the frequency can be badly shifted or erratic. Make sure the CPU load is low to nil (verify with "uptime" or "top").
You almost certainly need to be root (sudo or su) to access the GPIO pins (via /dev/mem) [ unless you already addressed this somehow, like via set user ID].
The added wire antenna is strongly recommended. Without it you might fail to pick up the signal.
If you had other processes that used the GPIO since the last reboot, they could be configured wrong. This can happen several ways. See: http://sourceforge.net/p/raspberry-gpio-python/wiki/BasicUsage for a basic outline of the issues.
Hard to know which of these is the issue, but I'll take a wild guess that it's root access or the lack of an antenna. Good luck!