4-bit shift register wont work (unknown state)
In a logic circuit, you typically cannot leave any pins floating (not connected to anything).

If you include preset and clear control lines, they need to be set to a logic level, either high (your logic-level voltage) or low (circuit ground). If they are left floating, then the flip flops are in an undefined state. When you remove the preset and clear lines from your simulation, then the flip-flops will operate normally because it doesn't expect any input from them. D flip-flops can come with or without these additional control lines.

Take a look at the truth table for D flip flop with preset and clear control lines.



The preset and clear lines are considered "active low" which means they perform their functions when pulled low. When they are pulled high, the clock and D control the outputs as you would expect. Notice, if they are both pulled low, the outputs are in an undefined state (represented by 'X').


Preset - sets the output (Q) high
Clear - clears the output (sets Q
low)