For QuickBooks Online (QBO) UK, there's only 4 filing frequencies available.
These are:
Monthly
Quarterly
Half-yearly
Yearly
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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