Best answer by Mohammed
Welcome to our iD Community!
There are currently 2 types of way data can be accessed.
The average download speeds on 3G is 6.1Mbit/s whereas the average download speeds on 4G is 15.1Mbit/s
What this tells us is, on average, 4G is faster than 3G.
Watching a video on mobile data would use the same amount of data regardless of whether you were on 3G or 4G.
So whilst this doesn't exactly mean 'more' data is used, it can certainly lead to it.
This is because the quicker you're able to use data, the more likely you are to go through it.
Let's say you're browsing the web on 3G, websites will tend to load slower (especially true if they have a lot more images/videos on them). So you'll spend a lot longer waiting for the website(s) to load.
Whereas on 4G, the website(s) would load a lot quicker due to the faster speeds, allowing you to get through more web pages (so to speak).
The best thing to do is to:
Monitor your data in the iD App to ensure you're not going over.
Set up a cap in the app (if you don't already have one) at the minimum spend cap to prevent you getting a high bill even if you were to go over.
Consider purchasing a data add-on if you've got little/no data left and still wish to use data.
Switch to WiFi wherever possible.
I hope this helps answer your question.
Do let me know if there's anything else I can help with!
EDIT: I've moved you topic to the "Coverage, international & roaming" section of our community to help others find the answer they're after