Is it difficult to enter the university after finishing the third year in Chinese mainland and the fourth year in Hong Kong?

It is difficult to go to university in Hong Kong. It seems that only 20% of your peers can get in (I don't remember the specific proportion, but it is relatively low anyway). However, if you are an excellent student in Chinese mainland, it will be easier for you to keep up with science after studying hard in Hong Kong. If you strengthen your English, it is no problem to get into college, and many new immigrants can get in.

If your grades in Shenzhen are average, it depends on how hard you have worked since you arrived in Hong Kong. As long as you decide to work hard, three years is enough. Hong Kong does not recognize mainland diplomas. If you want to develop in Hong Kong in the future, you'd better go to Hong Kong for an undergraduate course. If you really can't, you can apply for graduate students in the future. In addition, it seems that the score requirements for Hong Kong students to enter some designated universities in Chinese mainland are very low. You can do it yourself.