The phrase "a fourth occasion" is correct and usable in written English.
You can use it when referring to the fourth time something has happened or will happen.
Example: "This is the fourth occasion that we have met to discuss the project."
Alternatives: "the fourth time" or "the fourth instance".
On a fourth occasion, he was joined only by Justice Kennedy.
On a fourth occasion, the PM added, donors were present at a post-election celebration in Downing Street in July 2010.
On a fourth occasion, in 1824, candidate Andrew Jackson won both the electoral college and the popular vote, but the House of Representatives declared John Quincy Adams the winner.
On a third occasion (Oct . 97 most variables showed greatest significant variation at the quadrat-scale and the site-scale.
There was a second occasion when she expressed similarly scathing views about the EU at Buckingham Palace".
On a third occasion, Jonathan was taken to the hospital with severe bruising on both sides of his face.
The analyses were based on data from 2017 adults with two longitudinal occasions, of whom 948 had also completed a third occasion, with each occasion consisting of three parallel versions of the tests on separate sessions.
In a cross-over design, a group of 10 subjects underwent a MSLT (MSLT-P) following a PSG and, on a second occasion, a MSLT (MSLT-N) was performed without a prior PSG.
On a second occasion, in 2001, he said, Tahir asked him to transfer twenty-five thousand Swiss francs to Lerch's bank account, which he did without asking the purpose.
That is, on two occasions, high computer use coupled with high personality was associated with decreased perceived social support and on a third occasion this combination resulted in increased perceived social support.
