Allow me to clear this up for you.
For QuickBooks Online (QBO) UK, there's only 4 filing frequencies available.
No worries though, I have a workaround for you. Set your filing frequency to Quarterly, then when it's time to submit your VAT return, edit the ending date. Make it so the coverage of the VAT return will be 4 months instead of 3.
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Leave me a comment below if you have any other questions.