I can't see a problem you wearing a uniform of some form.
Number of bars is up to you, don't see a problem with four.
As Pace says the uniform does open doors and cut down grief most of the time. If not for anything else that you look as if you should be doing what you do.
And I also think its fair game for FO's to wear 4 bars if its going to get us an advantage. Any country where the FO can speak the local lingo and I can't its swapsie time. This has led to some very nice pizzas delivered to the aircraft in France, fuel bowsers turning up out of sequence etc.
What you might get though is the feds coming and asking questions about what you are doing just as folk on here have.
Ask the bloke thats hiring you what he wants, he might not like the idea or he may love it. You won't know until you ask and it will bypass the "what the hell are you wearing" conversation.