I introduced the dummy for all sleeps from 6 weeks of age. She is now almost 4 months old, when is it safe to wean the dummy?

There is strong evidence that dummies are associated with a reduced risk of sudden infant death when used consistently.
Mechanisms that provide this protection are not understood and pacifier use may possibly be a marker from something else, as yet not identified.
Red Nose does provide information about the use of a dummy, but has not included this in the 6 sleeping guidelines.

However, the information linked below provides comprehensive in relation to dummy use.

For those parents who do choose to use a dummy as a regular part of safe sleeping practice the following is suggested:

Parents who wish to use a dummy should do so only for sleeping periods and by the end of the first year of life dummy use should be phased out
Dummies should definitely be discontinued by 2-4 years to reduce the risk of dental malocclusion
https://rednose.org.au/article/using-a-dummy-or-pacifier