How can the Hadith, "Allah ordained the measures of creation fifty thousand years before He created the heavens and the earth," be reconciled with Allah's saying in Surat Fussilat, "And He destined therein its provisions in four days"?

The "estimation" (taqdeer) in the verse: {And He estimated therein its sustenance in four days} does not refer to predestination (qadaa’ wa qadar (definition)) in the sense of pre-decree. Rather, it means the actual creation and detailed formation, such as creating the forces from which sustenance originates, the origins of its various species, and determining the appropriate times for it.

Even if "estimation" were to mean the recording of destiny, there would be no contradiction. For the recording of destiny is of multiple types: a general recording before creation, a lifetime recording for the fetus, an annual recording on the Night of Decree (Laylat al-Qadr), in addition to the daily recording.