You are unlikely to be able to enter t4 from landside with a T5 boarding pass

You could enter T5 and then do a shuffle on the train from the departure stream into the arrivals stream, this will mark your ready to fly that you are in t5 departures

I am not sure if there is a direct shuttle to t4 at the moment or if it is still going via t2, so the transit each way could take 30 minutes or more

You may or may not get into t4 from airside.

When you return to T5 the barriers will think you are already in T5 and not let you back in. You will then need to find someone who can reset you in the system.