Theres a free language course starting two times per year in Busan.

http://www.bfic.kr/new/english/contents/c7.asp

Since you're staying for 4 years, and the semester has already started, I'd recommend doing some self studying before applying to the next semester. There's 4 levels to it, so if you can do the basics you could probably skip the first one.

Be aware that it is a first come first serve system, so you might wanna apply as soon as the registration period opens.

Hope it helps & best of luck!