Depends on the context, but T4 is a stage of cancer of the larynx and the "T" stands for tumor. T4 is the most advanced stage, with T1 being the involvement of the larnyx only... according to physician son-in-law ...
... and in line with the context of this category, film makers are in the process of making initial plans for the filming of Terminator in it's 4th resurrection, so in that context it means Terminator...