Adding 4-bit numbers to 16-bit shift register
The LOAD DATA input isn't just loading the high 4 bits, it's loading all 16. Since you have no inputs connected to the other 12 bits, the stored values are going to the unknown or 'x' state.

You can easily fix this by just using 4 4-bit shift registers. If the LOAD DATA input only controls one of the four, the other registers won't be affected when you load the first one.







simulate this circuit &ndash; Schematic created using CircuitLab

The circuit you have currently is equivalent, but with all the LOAD pins tied together.