I live in the UK as a UK national, and the US based company that I work for has bought me a computer in the USA to use for my work with them. I'm travelling to the US from the UK for business and to visit friends soon. They will give me the computer (Mac Studio) then before I return with to the UK. It will be their property and not for my personal use. I will have to return it at the end of my time with them, which has no end date (It is open). Do I have to pay VAT? It is worth $4500 in the US.
Yes, you have to pay VAT. If you just bring it in your luggage as is - the backstory is absolutely irrelevant.
A decent company would have a shipping department that would handle this for you. They shouldn't be giving it to you to deal with, they should ship it to you and declare it accordingly and cover whatever taxes the customs require.
If you decide to carry it, they still should provide the customs documentation. Unless it was a gift to you, it's their computer given to you for work. You're importing a piece of business equipment. See here for details.