4 weeks old is too young to be sold, but now you have the piggy, you need to be 'mum'. The pig will be timid & possibly withdrawn, especially if you only have one. You will need to make sure that he has everything he needs.
Before I go on, have you ever had guineas before? I am only asking what experience / knowledge you have caring for them? They do have specific needs which need to be met - fresh fruit & vegetables (although some are better than others), plenty of hay, water & guinea pig dried food. I don't want to tell you things you may already know!
The other question I want to ask is, do you only have the 1 pig? If so I would advise you to get a second. Piggies are compnay animals & thrive so much better when there are at least 2. That said there is always the odd loner, but at 4 weeks you won't know that. When I got my first pig Corran, no one told me I needed 2. When I was finally found out & got my Tycho pig, Corran became so much happier, chatty & social.
The other thing is use this forum! I have had piggies for 10 years now & I am still learning. There is usually someone here who can help! Don't be afraid to ask!
Michele & my quintet of manly piggies!
xxxxx