Plumbing Questions? Ask a Plumber for Answers ASAP
You need to reset the boiler by switching power off and then on.
When the boiler starts up do you hear / see the pilot light coming on, F4 points towards no flame being detected so would focus on checking the thermalcouple / electrode is working and that gas is getting to the burner.
Since this is rather involved it is advisable to get a gas safe engineer to attend, if you would like me to recommend such an engineer please provide your postcode.
Switch the boiler off any the isolation switch and switch it back on, let me know if you see the pilot light come on or if it comes up with a F4 fault straight away Silvia.
With regard to recommendation for a local boiler engineer I would suggest BB&PS on(###) ###-####8656 or SHS on 07784 067425, they both have excellent customer feedback, and would be the next step if the boiler shows with F4 after you switch it back on, since it suggests you need to strip the boiler down to check the gas flow, flame sensor and feed into the condensation drain.
Let me know if I can be of any further assistance.
Your boiler doesnt have a hole to view the pilot flame, but shows a picture of a flame on the digital display, to the bottom left can you see a flame symbol next to a symbol of a set of vertical bars?
This isnt an expensive part so when calling both look for the one which charges the least on hourly rate/call out