Well i'll post here to keep it on the forum so its searchable!
Theres two parts to the number you posted, the two digit code and the letter.
the two digit code gives you the weight the tyre can carry, and the letter is the speed the tyre is rated to.
To deal with the speed first:
V = 149MPH
W = 168MPH
Y = 186MPH
Z = "150+"MPH
So as you can see, as long as its V or higher your fine as far as the speed rating goes.
For load index you need to check your maximum permissable axle weights off the vin plate under the bonnet.
The number (ie 88) translates to a weight, you can see the table of weights here:
So if for instance the axle weight is 1200kgs, your tyres need to be able to support 600kgs each, and therefore the load index as an absolute minimum would be 90. Most manufacturers add a 10% margin to the rating, so they'd choose a rating that would handle 660kg, which means they'd pick 94.
I think most A4's are fitted with 91's from the factory, although some of the heavier cars like S4 Avants might need 94's
Check the vin plate, and if your not sure post the numbers up and i'll tell you what you need.