To be frank I cannot see it sitting comfortably with a Z4 for many reasons. Space, performance and basic purpose of a Zed.
The tank takes space, so is often put in the spare wheel or slung underneath - neither work so you'd have to sacrifice the already small boot
Fuel economy is probably less important than performance so with LPG dropping the engine output and adding weight it's a double whammy
Lastly few Zeds do the mileage to justify the cost benefit equation. Big 4x4's do as do high mileage taxis, etc.
I love LPG BTW and have one of my big V8 Jeep engines running on it with a 110litres tank and it's real neat. I used to get 15mpg and this gives me the equivelant of 25-28. The engine still runs on its side and you can't do that with a carb set up
There are a few Germans on zroadster.com who have done LPG conversions.
Here's an example: http://www.zroadster.com/forum/bmw_z4_roadster/45950-z4_3_0i_vialle_lpi.html .
Personally I believe it's not worth the sarcrifice in trunk space, weight and handling.
I have an Alpina B10 3.2 Touring running an LPG with a 74 litre tank where the spare tire sits normally. Runs fine and you don't loose any trunk space.