To find a suitable alternative, you need to consider what you're using the TH400 for and what you need from a 4-speed. Options depend heavily on the application (vehicle, powertrain):
* For high-performance applications requiring heavy-duty strength: You might consider a built 4L80E or 4L85E (GM's electronically controlled 4-speed automatics). These are significantly more complex and require more sophisticated controls than the TH400, but they offer more gears and potentially better fuel economy. Building them to the same level of durability as a built TH400 may be quite expensive.
* For lighter-duty applications: A 700R4 (GM's 4-speed overdrive automatic) might be considered, but it's not nearly as robust as the TH400. It's more suitable for smaller engines and less demanding applications.
* For a manual transmission alternative: A 4-speed manual transmission is an entirely different animal, but provides a different driving experience and may be more appropriate depending on the desired performance characteristics and application. This would require a complete change to the drivetrain setup.
In short, there's no single "drop-in" replacement. The best alternative depends entirely on the specific application and the desired performance and durability characteristics. You'd need to consider the engine's horsepower and torque output, the vehicle's weight, intended use (street, drag racing, off-road), and budget.
