Can my mid 2011 imac 27 inch be used to display my ps4 using a hdmi adapter or something else?

Here's Apple's page on how to use Target Display Mode. As you will see there is no option to connect from an HDMI port to a Mini Display port.

Use your iMac as a display with Target Display Mode - Apple Support

No. Not with any adapter.

I think the 2010 27" iMacs were the last ones that could use the Kanex XD adapter:

Amazon.com: Kanex XD HDMI to Mini Displayport Converter- iMac 27 inch to HDTV: Electronics

If you have an XBOX you could try the OneCast.app to stream to your iMac. Not sure if there is a similar streaming app for PS4. Streaming is kind of laggy.

https://onecast.me/

The article is from almost 9 years ago. If you want to try it go ahead. To my knowledge it won't work.

Your iMac is equipped with a Thunderbolt port and not a display

port and can only be used with another Mac with a Thunderbolt port.

Even though it can output to a display port compatible monitor,

it cannot be used as an input.

2010 iMacs were the last ones that utilized the display port technology.

Even those models were dicey in terms of getting them to work in target

display mode with anything other than Macs.