Connect 2 line phone to a 4 line system
This question cannot be answered without a lot more information regarding the multi-line phones. 

Many later model business telephone lines use different interface protocols between the phones and control unit back in the telephone equipment room than the typical "tip and ring" connections used by a "cheap two line phone". 

The business phones may be using digital signalling or a combination of digital and analog signalling. The the "cheap" phone will be using the analog type of signalling.