How long into the fiancé visa did you get married and apply for spouse visa?

If I were to leave 4 weeks before her fiancé visa runs out then that should not be a problem?

Shouldn't be an issue...

My wife's Fiancée Visa was 'Issued' on 21 August 2018 - she arrived in the UK on 10 September 2018 and we got married on 19 January 2019. Her Fiancée Visa Expired 21 February 2019 (6 'Calendar' months from Date of Issue)

Just checked her first FLR application - dated 28 January 2019, so 9 days after our wedding day and approximately 3 weeks before her FV expired. We must have received the Marriage Certificate by then!