You can use a variety of approved study providers where you can purchase material for F4. Most commonly people go to Kaplan, BPP, LSBF. However, you can also access free material from Open Tuition, where they have course notes along with lectures and practise questions which I have found to have been very useful.
The link for Open Tuition should you wish to have a look is here: http://opentuition.com/acca/f4/
Hope that helps.