Hi, my partner just received tourist visa for the UK. On the application she said she was only visiting the UK for 2 weeks. Now she can go for 4. Is that something to declare to UKBA?

She can visit for any length of time within the validity of the visa.

If she hasn't visited yet there's no need to advise the UKVI of her change of plans.

When she arrives at the UK Border she might need to advise the Border Force Officer of her change of plans, if asked, but that would be unlikely.

She might want to mention the fact that she changed her plans in any future visa application, especially if she used evidence of a two week leave of absence from any employment as a reason to return.

To be quite honest I doubt if four weeks instead two is going to cause problems in any future application, it's more likely to be an issue when an applicant stays six months instead of a couple of weeks, especially if they claim to be employed, though doing so doesn't breach any Immigration laws.

Ok, thanks I just wanted to make sure she doesn't get refused entry because the dates were different to what was on her evidence letters (from the educational office) even though it's within the 6 months. Glad she can stay for longer now.

If she has been granted an extra two weeks leave, then it may be an idea to get a letter confirming this.

Then in the unlikely event of her being questioned when entering the UK she can show this to the officer.