I interpreted this as: "I want to complete my fourth year of my undergraduate Computer Science degree in Japan after LEARNING the Japanese language (but it would be great if there's an English undergrad program) and the continue on to post graduate study in Japan."
I'm sorry, is that accurate?
First, there are a couple universities with undergraduate programs taught in English, but I don't know of any related to Computer Science. Temple University Japan Campus has something media related I think...
Second, for an undergrad course you're going to need to take the EJU and get a score of at least 219, and as you're transferring they may want a bit higher score. If you haven't studied Japanese before, you'll want to add in 2 years at language school.
Third, You'll probably have to start as a Junior (3rd year) if you transfered to a Japanese university.
Fourth, Tuition is relatively cheap at Japanese universities, but living expenses are expensive and you can only work like 4 hours per day on a Student Visa.
If you only have one year left at your home university, it might be easier and more beneficial to graduate from there first and then go to Language school, then Grad school in Japan.
Good luck to you in whatever path you choose,
a7mouse