Try the Audi A4 forum, as the likelyhood of someone else having tried an A4 with various tyre/wheel comparisons on here is very low...
www.vwaudiforum.co.uk/forum/forumdisplay.php?24-A4...4

Lol, firstly, changing wheels/tyres does not affect warranty.

Secondly, smaller wheels will obvioulsy fit over the brakes unless the brakes are mods or he goes for 14" wheels.

Yes, you should inform your insurer (not ask for their approval, THEY work for YOU, not the other way around).

Do not change cars because you don't like the wheels/tyres, that would be ridiculous.

Edited by Bronan the Brobarian on 15/05/2018 at 18:24

The OP can easily check for compatability by referring to the plate on the inside of the driver's door or the owner's handbook, which should give which tyres and wheel combinations can be used as standard with the car.

I just changed those on my Mazda3 from 205/55 R16V to 195/65 R15H which was allowable by Mazda (plate and handbook, and I checked with my dealership), and I also checked with my insurer and it was fine, well, better than fine - I actually got a reduction in my premium of £15 and (as a long standing customer) they waived the fee for changing my policy mid-year (normally £25).

In addition, the new, smaller alloys (still the Mazda OEM design) were nearly £60 cheaper, each, and the tyres cost about £20 each less before a lucky discount. £320 saved on replacing all wheels and tyres (less than £600 including fitting [which I didn't have to pay for in the end - another story] as opped to well over £900 for 16in versions).

Ultra low profile tyres are blimmin' expensive and don't last long - something a LOT of premium car buyers (especially second hand cars) don't always realise. The 18in tyres fitted to the latest gen Mazda3 cost £130+ because they are an unusual combo of width and sidewall percentage. God knows how expensive these Audi ones cost.

The OP may be able to make a decent amount of money back if they sell the 4 existing wheels and tyres to some mug wanting them as posing bling for their Audi, to offset the cost of buying new alloys and tyres, assuming they will fit (as skidpan says) over the existing brakes. Two sizes down seems rather a lot to me for compatability - one, perhaps, two...