'Section 18' is an extremely serious charge. (It carries a maximum penalty of life imprisonment).
The 'starting point' for sentencing of the least serious Section 18 offences is 4 years imprisonment, with judges required to sentence in the range of 3 to 5 years unless there are good reasons not to do so. (A guilty plea, from a first time offender, at an early stage might get the sentence reduced rather lower).
However, Section 18 is 'GBH with intent'. A cut requiring 4 stitches does not amount to 'GBH' (and thus 'with intent' doesn't come into it). The only exception would be if the cut resulted in permanent disfigurement (which would have to be 'more than minor'). A decent barrister should be able to get the charge dropped to Section 47 (ABH), with the likelihood of a non-custodial sentence (unless there's a past history of violence).
Read through Sections 4 and 5, here, to see why I state that ABH should be the correct charge:
http://www.cps.gov.uk/legal/section5/chapter_c .html#09
Chris