I'm portuguese arriving next Sunday to HK. Can I get a Mainland VISA from HK in 4 days?

You can try to apply in HK, but it is not assured that you can get it in the end. You are suggested to find a local tour agency to help you. Usually, the processing time is around four working days, but for exact time, you need to consult the agency directly.

You will apply for a Tourist type, right? If so, you still need to provide your round trip tickets to mainland, hotel booking, two photos, an application form, your entry slip for HK.