Tier 4 visa
If you have a minimum overall IELTS score of 4.0 (with no sub-tests less than 4.0), you should apply for a Tier 4 visa. You can obtain a visa that covers the duration of your English language course and degree programme. However entry on to the degree programme is conditional upon successful completion of the English language course.
Upon successful completion of the English language course, you may apply in the UK for the Tier 4 visa for your degree course, as long as there is not a gap of more than 3 months in between courses. The gap between the end of your current visa and the start of your new course must not be more than 28 days.
Short term student visa
If you have an overall score of IELTS 4.0 (with sub-tests less than 4.0) you could apply for a short term student visa and you may study 30 weeks of English.
Upon completion of the English language course, you must leave the UK to apply for the Tier 4 visa for your degree course from your home country.
Further information on the differences between a Short term student visa and Tier 4 visa.