Subaru dealer should be able to provide the 4 digit code if it's a standard UK Sigma alarm. Just prove to them you're the owner (V5 etc).
On the current problem, try disconnect the car battery for about 30 minutes. If the alarm sounds immediately, try the remote button (may help if it's close to the car). If no alarm sound, depress the brake pedal a couple of times to bleed any residual current, then reconnect the car battery. Then try the key button,