I have an old IMAC version 10.4.11 Can I use it as a display monitor only and how do I connect it to a MacBook Pro circa 2015

What you are asking for was called Target Display mode. As 10.4.11 was from 2007 and earlier, the answer is a definite no your old Mac will not support TDM. I think you will find experienced users like Allan and myself suggest that you simply buy an external display for your MBP. They start at $130 and are simple to setup and use and are far more reliable than TDM ever was.

You've limited the information we need to effectively answer. What OS in on the 2015 MacBook Pro running?

"Old" does not narrow down the iMac model enough to even begin to recommend how to connect. Chances are that, if running 10.4, it is far older than the range of iMac capable as being external monitors under Target Display Mode. The OLDEST iMac that can be an external monitor shipped with macOS 10.6.1.

Supported models and OS versions here:

VIDEO SOURCE COMPUTER (the one sending the video signal to an external monitor):

• A 2019 or older Intel Mac running macOS 10.15 Catalina or older.
• Apple Silicon Macs are not supported
• mac OS11 Big Sur is not supported.

TARGET COMPUTER (the one to be an external monitor):

• An iMac 27-inch (Late 2009 and Mid 2010)
• any iMac from Mid 2011 to Mid 2014.
• The target computer must run no higher macOS than OS 10.13.X High Sierra.

Use your iMac as a display with target display mode - Apple Support