The tyres with the most tread are at the front. The garage said that all weather tyres should be changed at 4 mm same as winter tyres?

To paraphrase Mandy Rice-Davies, "they would say that wouldn't they".

If you're expecting to use them mainly on snow, then certainly replace them at 4mm. Otherwise it's your money you're throwing away for the tyre fitters to flog off your old tyres to these 'used tyre' places

If a new tyre has 8mm of tread, then you'll get around 50% more miles out of them if you replace them at 2mm than you would by replacing them at 4mm.

Myself and, undoubtedly, the majority of folk on this forum manage more than adequately on Summer tyres all year round.

The tyres with the most tread are at the front. The garage said that all weather tyres should be changed at 4 mm same as winter tyres....

I trust your legs are still the same length as each other.

The tyres with the most tread are at the front. The garage said that all weather tyres should be changed at 4 mm same as winter tyres?

The garage is talking utter bunk, more likely they want to see you some tyres. IMHO service outlets also charge a BIG mark-up for tyres - mine wanted £85 (fitted) a corner for tyres that I actually sourced (from a reputable online retailer, incl. delivery) for £52 each, and (ironically) my service agent charged just £10 each for fitting, so I was quids in to the tune of £92 overall.

The legal minimum is less than half that, but most often tyres should be changed because of their condition - damage, especially to the sidewall, significant cracking, which is often due to age (newer designed quality tyres should last longer than ones from 10 years ago) or the ageing effect of the car sitting in the sun a lot, as well as just wear - which includes significant uneven wear caused by under/over inflation and/or tracking/alingment issues.

The moral of the story here is to have your tyre rotated (most service agent will do this FOC at the service). With directional tyres (which most all season ones are nowadays [your may not be though] - my Michelin CC+ are), all you can do without completely refitting the tyre to the hub is swap the fronts to the rear (but staying on the same side).

As long as the wear is relatively even (some cars have inherrant problems with this even where the tracking is ok) across the width of the tyre (see above for why it may not be), then doing this should extend the life of your tyres overall by a minimum of 50% if not 100%.

Keeping the wear even is a must if the car is a 4WD one, as the system will not work well (and may stop working) if the wear difference is more than (if I recall) about 2-3mm between fronts/rears.