Kempo

Noticed on the weekend that one of my front tyres has a bulge from a recent pothole encounter. All the tyres have 4-5mm so would have another 12 months or so with my mileage but have been reading that for AWD you need to replace the tyres in a set so there are no big differences in tread depth that can cause damage to the diffs.

Should I replace all 4 given how low the tread is. New tyres are around 8-9mm I think.