Try reading the official gov uk guidelines.You can also drive out of tier 4 into another tier simply to go for a walk or drive. But the media will tell you otherwise!
The words you must not surely means you cant.
You are only allowed to leave the place where you live for valid reasons.. don't see going for a walk as a valid reason to drive between tiers.
https://www.gov.uk/guidance/tier-4-stay-at-home?priority-taxon=774cee22-d896-44c1-a611-e3109cce8eae