The four had started work within the previous two weeks at hospitals in south and west Wales
Cardiff University is changing the way its medical exam results are produced after a mistake led to four students graduating despite failing an exam.
The four were told they had passed their finals last summer and had begun practising as doctors before the mistake was uncovered.
They were suspended from duty and are now repeating their final year.
The changes include two new staff and bringing forward the final written exam to give more time to verify results.
The university has personally apologised to the students involved in last year's mistakes.
The four had begun practising at hospitals across south and west Wales, but only for two weeks or less, and all were heavily supervised, according to the health trusts involved.
Cardiff University set up an investigation following the mistake which made a series of recommendations to prevent a similar error in the future.
In a statement, it said it was introducing a series of "robust measures" to improve the way student assessment data was managed in the school of medicine.
Two new posts have been created to oversee the new procedures - an assessment data manager and an academic psychometrician.
A new director of medical education has also recently been appointed.
The date of the final written exam has been brought forward to May to allow more time to verify results, and a monitoring group has been set up by the university to oversee the implementation of the recommendations.
Dean of medicine Professor Paul Morgan said: "The school of medicine recognises the seriousness of the issues which have been raised through the investigation and I reiterate my personal apology to those students affected.
"The school will be undertaking all the action required to ensure that such errors cannot be repeated. The implementation of the recommendations is regarded as an absolute and high level priority.
"The recommendations in no way relate to the high quality of medical teaching at the university where students benefit from interaction with researchers who are working at the forefront of medicine."
The General Medical Council (GMC) said the university had "significant progress to make", but it had "begun to address these issues by introducing a series of measures to ensure this situation doesn't occur again".
The GMC has asked for a "detailed action plan" and wants assurances that "there will be no repetition of these errors which had extremely serious consequences for the students involved".