How can I model a relay in LTSpice IV?
As @KevinWhite said, you can use a voltage controlled switch. Here is an example of a very rough model for a relay:



And this is the result of the simulation:



The model is very rough because:


The excitation coil is modeled only by a resistor, neglecting the inductive component (this may be OK).
The switch triggers when a voltage is applied to the terminals of the coil, whereas physically the relay triggers when the current in the coil is sufficiently high to move the contacts.


Both points above can be neglected unless you need to simulate the exact behavior of the relay under transient conditions (switching delays, oscillations, contact bounces, etc.). 

An inductor in series with the coil resistance can be useful to simulate the inductive kick the relay produces when switched off.