Posted 06 July 2015 - 06:15 AM
If your iMac is running Tiger (aka Mac OS 10.4), then it does not support Target Display Mode.
The only way you could then use the iMac as a monitor for your Mini would be to do screen sharing of some sort across the network. The downside of this approach is that it can be VERY slow as you are "remotely" logging into the Mini from the iMac.
You would likely be better of just buying another monitor most likely.
The other possible option is to hook the monitor that you do have up to both the Mini and the other computer. This assumes those two computers are in the same room near each other. If the monitor has dual inputs then you would not need anything else. If it does not have dual inputs, then you could buy and use a KVM (keyboard, video, and mouse) switch.