Can I use my2019 iMac 27" (3.7GHz i5/Radeon Pro 580x) as a monitor for my MacBook Pro M4 Max?
Can I use my2019 iMac 27” (3.7GHz i5/Radeon Pro 580x) as a monitor for my MacBook Pro M4 Max?
MacBook Pro 15″
No.
That setup is not supported by Apple. The old iMac cannot be used as a monitor for any of the newest Macs and Target Display mode is no longer a feature of macOS, having been deprecated years ago.
Please see: Use your iMac as a display with target display mode - Apple Support
Target display mode is dead tech.
Third-party solutions such as Luna Display or Duet Display are performance limited and suited only for uses where there will be limited motion and where lag time is not an issue, such as for application tool palettes. You still will need a proper monitor to set up either of those options.
Do yourself a favor, avoid any headache and just budget an extra $100-$300 for a monitor. Yes, they are that inexpensive.