There are 2 ping sensors between cylinder 1 and 2, and the other one is located between cylinder 3 and 4. The exact place the located should be at the height of the pistons, outside ( :P ) the engine.
I do have this information from the 'Bosch Motronic' manual, where is be written about this ping sensor position for 4-cylinder engines. Also for this BMW engine I guess!
Good luck!

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