Just for comparison...........................
My B5 S4 Headlights had 4 bulbs. [2 x Dipped HID/ 2 x Main Halogen].
I modified the Halogens (Mainbeam) to HID.
My B8 S4 (Bi-Xenon] Headlights have only 2 bulbs [HID].
B8 A4/Allroad [4 Bulbs]:
H7 Halogen = 1,500 Lumens each [ X4 = 6,000 Lumens Total] (3,000 Dipped AND 3,000 Lumens Mainbeam)
B8 Bi-Xenon [2 Bulbs]:
35W HID = 3,200 Lumens each [X2 = 6,400 Lumens Mainbeam Total]
55W HID = 4,000 Lumens each [X2 = 8,000 Lumens Mainbeam Total]
My B5 S4 (unmodified) = 6,400 Lumens Dipped AND 3,000 Lumens Mainbeam [9,400 Lumens Total].
My B5 S4 (modified) = 6,400 Lumens Dipped AND 6,400 Lumens Mainbeam [12,800 Lumens Total]
My B8 S4 (unmodified) = 6,400 Lumens Dipped OR 6,400 Lumens Mainbeam. [6,400 Lumens Total]
My B8 S4 (modified) = 8,000 Lumens Dipped OR 8,000 Lumens Mainbeam. [8,000 Lumens Total]
Remember Bi-Xenon HID has the bulb partially obscured by a "shutter" on Dipped and shutter moves away on mainbeam.
On the B5 conversion this meant there was a "warm-up" period each time the mainbeam was lit (unless under 20 seconds since last on).
D1S far more reliable (and able to run 55W) than D3S
D1S Bi-Xenon were also used in B7 RS4.
This does not take into account the relative efficiencies of the reflectors or projectors used and variances in different quality bulbs and ballasts which are available.