I think Nodding Donkey is spot on here.
if you have the legal hours to work elsewhere as long as the second job is in a different sector ( not in direct competition) and you’ve been a model employee no time off, work completed, not late, always do whats asked of you, i cannot see how your main employer would have any grounds to sack you?
Surely they would have to prove your second job was affecting your work before they could do anything. As long as you do all that’s asked of you its not harming anyone. Employers do not own you.
Unless it exclusively says in your contract that you must not work for another employer, the wording on most contracts is usually open to interpretation anyway, i can’t see that doing a 4on/4off role should stop you from doing a bit somewhere else if you want to!