What is the weight of the car trailer? The BMW may weigh about 3000 lbs. If the trailer is more than 2000 lbs then you need to find another tow vehicle. Also, anything over 3000 lbs you need to have electric brakes with an electric brake controller. Also you my have to have a load equalizing hitch if your hitch weight is too high. May not want to go over 500 lbs pushing down on the hitch. Another point to consider is if the trailer is fairly long your 4Runner wheelbase may not be long enough even with an anti sway control hitch to safely tow. General rule of thumb:
For the first 110" of wheelbase, this allows you 20' of trailer.
For each additional 4" of wheelbase, this gets you 1' more of trailer." Review the manual and or ask a Toyota Service Director for advice.