How long was Shannon in jail?

Shannon was in prison "since yesterday" - it does sounds like "six years today" due to Dr. Okoye's accent. You could see it was immediately after the beating they gave him, that slop pan/urinal next to him is full of blood from the damage to his kidneys.

The doctor said he had been imprisoned for 4 years but probably wasn't subjected to Shannon's brutal treatment.

Probably less than a week, though it might have felt like longer to him, especially if he was being beaten up regularly. It may not have been his first experience of jail. When he sees his doctor after being deported you have the impression this is not the first time he has come back from somewhere in an injured or knocked-about condition.

"Chicken soup - with a *beep* straw."