There are several reasons:
First as the saying goes, a machine is only as capable as its operator. Can Turkish pilots stand up for the job? Or will they be beaten up on their streets as has been recently reported that Turkish F-16 pilots are being beaten up on the streets of Turkey by Arab diplomats (for example one Turkish F-16 pilot who is actually the Son-in -law of Turkish Chief of Airforce was reported to have his nose broken by a Kuwaiti diplomat on a Turkish street in front of his wife, therefore losing his flying status).
Secondly an F-4 despite its age, can carry much bigger a bomb load than an F-16. For a ground run mission, bomb load is more important than aerial maneuverability in ground missions.
Thirdly, F-16 is a single engine aircraft. A mechanical failure above enemy territory will cause a huge embarrassment for mission planners as they lose an aircraft and will have to deal now with a captured pilot. F-4 has two engines and therefore, there is less of such a probability.
Fourth, you have to achieve your strategic goals using the tools available. F-16 is not available to Iran, but Iran has been able to achieve its strategic goals by using F-4's. Has Turkey been able to achieve its strategic goals using its F-16's? The answer is clear.