The XR4Ti came from the factory with Ford's 2.3 turbo powerplant, which was also used in the Thunderbird Turbo Coupe and Mustang SVO. It produces around 175 hp stock in manual transmission trim and puts it to the ground via the rear wheels.
The Merkur XR4Ti is a variant of Ford's (European) Sierra platform. It is most similar to the XR4i model. Minor changes were implemented when Ford brought the XR stateside: namely, it got 5 MPH crash bumpers and the V6 was replaced with a 2.3L turbocharged four. The change in name is allegedly due to GM's usage of Sierra on its truck models (or the Cutlass Ciera.)
The name XR4Ti was given because the car is a member of Ford's higher-performance family (XR), it is turbocharged (T), and it has fuel injection (i). The "4" is just there to differentiate it from other XR models (for example, the XR3i is an Escort.)
