I have a Surface Pro 4 and an iMac (late 2019). I would like to connect my Surface Pro to the iMac to use the iMac screen as a second monitor. Is that possible?
What are the requirements to use an iMac as a second screen?
No. You cannot do this. The 2019 iMac is not supported for Target Display Mode.
This is an attempt to write a canonical QA for this issue, as per the Meta post: Where is the list of canonical questions stored for Ask Different? I expect it to be periodically edited with the goal of becoming a comprehensive information resource.
General Rule of Thumb: If it's after 2014, Target Display Mode isn't supported.
24 and 27 inch iMacs from 2009 - 2010 support TDM via Thunderbolt or mini DisplayPort (mDP)
All iMacs (21 inch and 27 inch) from 2011 through mid 2014 support TDM via Thunderbolt only.
Retina iMacs from late 2014 and on do not support TDM at all.
All Mac models through 2019 that meet the above requirements can use Target Display Mode. Currently, 2020 Macs will not connect (i.e. 2020 MacBook Pro running Catalina), but a 2019 MacBook Pro also running Catalina works fine.
Yes! As long as the target Mac meets the above requirements and the PC has the proper port; Thunderbolt or mDP. The OS on the source computer doesn't matter at all.
No. Your iMac must be a fully functional iMac running High Sierra or later.