Here's what you need to consider:
* Bolt Pattern: The bolt pattern on the bellhousing should be the same. However, slight variations are possible depending on the engine and year.
* Output Shaft: The output shaft diameter and spline count must match the transfer case in your Suburban.
* Electrical Connections: While the basic functionality is similar, there might be differences in the wiring harness connectors. You might need to adapt the wiring.
* Torque Converter: Ensure the torque converter is compatible with both the transmission and your Suburban's engine.
Recommendation: Before installation, compare the specifications of the 2004 4L60E you have with the specifications of the original transmission in your 1999 Suburban. A transmission specialist or a reputable mechanic specializing in GM vehicles can help you determine compatibility. Simply assuming it will fit without verification could lead to significant problems and unnecessary expenses.