Roof rack and bike carrier recommendations

I have a Thule roof rack system with 3 bike carriers, mine uses 1200mm aero bars which have now been replaced by the aeroblade/wing bars. Each time I change cars I just buy new feet and sell the old ones. I think all Thule bars essentially have the same clamping system to the car and whatever carrier adapter you want to use. The aero bars can whistle unless you fit the blanking rubber inserts, but that's only a one off 10-min job and once the bike carriers are on then everything stays as is.
I remove the roof bars and bike carriers as one unit, this obviously needs two people, but its way quicker than assembling everything from scratch each time on the roof; to fit or remove as one unit like this takes about 10-min.
We have three carriers on the bars, the centers of which are 18" apart and the carriers are arranged front to back so the handlebars do not impede each other (we have mountain bikes, road bikes are obviously narrower). To get 4 on might be a bit tight so you may be best laying things out somehow before buying the right length bars for 4 bike carriers.
A far as recommendations go the bike carriers we have I don't think are made any more (Thule 510) I've attached an image I've found off the internet of what they look like, they don't look too dissimilar from the FreeRide, as they have an adjustable jaw so they are good for both our road and mountain bikes. Thule stuff is not the cheapest, but they are quality items, hence why we've had them a good few years. We've been to France and back a few times as well as using in the UK and the bikes are very stable.
I've not got a photo of the three bike racks on the A4 only one, but I do have one of all 3 on my old C350 for comparison, so you should get an idea of the A4 roof clearance (this will be essentially the same whatever Thule bars you use as the feet determine this) and spacing on the Merc. I'll have one of the full set up once I take my son back to Uni in a wee while.
Hope this helps

Be aware that roof bars/racks eat your fuel consumption. Manufacturers spend millions developing the aerodynamics of the car only for it to be spoilt by the parasitic drag of a rack. It could cost you 10% of your fuel consumption. I appreciate some are tricky to get on and off but the fuel saving must be worth the effort if it's not in regular use.

MOA4B9 said:

The roof bars are the most aerodynamic ones so should reduce drag. I think that at slow speeds like around town they don't cause that much drag. Drag increases disproportionately with increase in speed.

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You are correct, drag increases by the square of the speed. Double your speed from 20mph to 40mph and there is 4x the drag. Again from 40 to 80, the same, so 20mph up to 80mph, you increase the drag by 16 times that at 20mph.

Only mentioned as some people are unaware of the huge penalty on fuel consumption.

Just wanted to see if I could add my input for your search.

Bike carriers: I had a 2 bike Thule model (tow bar fitting) that was okay but then water got into the connections and ruined it. I couldn't get any spare parts for a 4 year old model. I live in an area in France that has low rainfall and rarely used the carrier in wet weather. Based on this experience I found another make called 'Atera'. Made in Germany and I bought it, pleased with it after 1 year. I think many people buy Thule as they are everywhere but in my opinion they are not the best.

Roof Box: I'll be buying one before Summer and after lots of checking around I have decided on the model Hapro Traxer.

I think this company online do a good choice and offer good service www.roofbox.co.uk

PS: I think the only way to hold bikes properly is with a tow bar mount.