So have you been an overstayer here in the UK for the last 4 years? If so and you want to now continue to be a student then you are going to struggle. In October 2008, the Home Office brought in a re-entry ban which meant that anyone in the UK after this date who did not have valid leave to remain would be banned from coming back within a certain timescale except for in certain circumstances.
If you were in the UK illegally after this date and you left voluntarily at your own expense, you would have just a 12 month ban from returning.
If you left at the Home Office's request but at your own expense then it was 5 years.
If you left at the Home Office's expense and request then it was 10 years.
These bans only apply if you are coming into the UK as an economic migrant (ie to work or study) and do not apply if you are coming on a family visa.
As you are an overstayer, it is not possible for you to make an in country application as this will most likely be refused if you do and you will lose your fee.
If you return and immediately make an out of country application, the Home Office will most likely notice the dates in your passport and refuse your application on this basis.
My advice would be to return voluntarily and at your own expense and assuming you have no family in the UK to return to, wait 12 months and then make an application to come back with the help of a solicitor to ensure that your application is fully explained and meets all the rules.
Spend the 12 months working and building up savings to ensure that when you make another application to return, you will meet the rules.
If you have family, you could of course make an application earlier and if it is refused on the basis of the ban, appeal the decision.