How long have you had her? Has she grown much in the time you've had her? This will give you a pretty good indication of age... if she's grown quite a bit in the time you've had her, then she's still fairly young and will probably grow more. If she hasn't grown much, she's probably an adult and will still continue to grow but very slowly.

On that note, a 5'8'' snake should be perfectly comfy in the 4' cage. 6' cages are normally used for larger, breeding females.