Did a couple of quick googles for some info to make sure I had updated figures
The average potty training age varies widely around the world, ranging from 6 to 18 months in parts of Asia, Africa, and rural communities, to 2.5 to 4 years in Western nations like the United States, United Kingdom, and Sweden
The average age for potty training in the UK is around 2.5 to 3.5 years old
So the 3.5 year old is getting towards the top end but not unheard of.
Children should start weaning off milk bottles around 12 months of age and be completely off them by 18 months , according to health guidelines from the NHS and pediatric specialists
Looking at average age for the UK it said
Many toddlers continue using a bottle for comfort—particularly at bedtime—well into 15 to 18 months, and sometimes up to 2 years.
This means your 4 year old is very past when the recommendation are and the average.
However whether either of you should comment on the others way of parenting is a completely different question and one IMO where you both should stop (though I acknowledge you didn't comment you had mentioned the potty training)