I am no expert either gnu, but from Syria's POV it is not so strange.

Less than 5 years ago Syria's nuclear facility was completely destroyed by an air strike, humiliatingly without the loss of a single aircraft.
(and are we not all grateful to Israel now?)

That was a powerful incentive for them to acquire a world class air defence system.

Turkey has been outspoken against Assad and has been harbouring insurgents.

Syria wants the world to know that in a Libyan style air intervention it will not be such a push over.

If there was such an intervention, it would come from Turkey and Turkish F4s would be a part of it.

It was flying fast and low and penetrated Syria's air space.

It is disputed where it was when attacked, but it crashed in Syrian air space.