That would be 4 days or however many hours you were there for.
Each university has it's own requirements and they can change, some universities prioritise more than others. Some also require a set amount of hours in certain settings such as clinical Vs non-clinical or small animal Vs large animal. It would be worth you maybe having a look at the entry requirements for the ones you're potentially interested in and going from there. RVC & Aber both wanted 140 hours minimum & 50% needs to be clinical.
Even if a university doesn't require work experience hours they're still really beneficial and will help you prepare for interviews. Placements in a vets practice can be a little challenging to find in some areas so perhaps worth prioritising securing those sooner rather than later.
It's important to know when the work experience has to be completed, for most it's within a certain time frame before you apply. I believe its 18 months but could be wrong, not sure if that's relevant to you but worth keeping in mind.