It would depend on what is written in your terms of employment, but as per the above it would be at least 4 working weeks - so 16 days (as your working week is 4 days).
It may work out to be 4 weeks, but mightn't. The other options for calculating holidays (which the employee can choose between) are:
Based on the employee's working hours during what is called the leave year, which runs from April to March. An employee who has worked at least 1,365 hours in the leave year (that is, an average working week of 26.25 hours) is entitled to the maximum of 4 weeks' annual leave. Many employers use the calendar year (January-December) instead of the official leave year to calculate entitlement
By allowing 1/3 of a working week for each calendar month in which the employee has worked at least 117 hours
Neither of the above applies to OP because of the hours worked - so the 8% calculation is the only one open to OP. So OP is looking at 1.44 hours per week holidays (18 * .08). 74.88 hours per year - 4.16 weeks per year, but subject to the maximum of 4 weeks - so I ended up where we started and it works out at 4 weeks after all! OP, note that it's 4 of the part time weeks, not 4 X 40-hour weeks.