Misfire with no checkengine light - SOLVED!

Hi all subarus lovers! I'm new here!
I'm an Italian living in Spain, and I make the big mistake of buying a Subaru Forester 2002 in this country where is very difficult to found someone that know this brand.

As I sayed I have a Subaru Forester 2002 SF, with a 125 no-turbo engine with a LPG system.
I bought this car in november, I love this car and I bought it at cheap price but with a misfire. At the beginning it was a check engine light, but at the first trip (dealer --> my house) the check engine stopped.
I leaved it in a friend car workshop and the man didn't do anyithing for like 2 months, so I take bring the car to my house and trying see what's going on.
My workshop change the knock sensor and he sayed to me that the car worked fine.. NO WAY.. the car is the same as before..

There is no CHECK ENGINE light, but one cylinder is misfiring.
I have changed all the sparks plugs but the problem is still there.. so I tried (with engine turned on) by desengage the wires from the ignition coil of any cylinder one by one.. and I see that only Cylinder 4 is failing.

So I think "maybe it's an ignition coil issue!".. so I bought a new ignition coil.. and today I discovered that the car have the same problem... I didn't solve anything.

I can't do the diagnostic because I don't have the correct OBD adaptator..

What issue can be?? Only one cilinder so it can't be the cam belt wich is bad installed. Maybe a valve clearance??

HELP! and sorry for the bad english

PD: When I changed the plugs sparks I notice that they are all with carbon on the top.

VIDEO OF THE TEST: I can't put the link because I am new and the system didn't let me.

Thank you!!