Joomla 3 to Joomla 4

Hi Team,

I am wondering if you can assist me, the website of my company was built with a template called jp-perfect2 Default we are in Joomla 3. We need to upgrade to Joomla 4 and 5.

After the update to Joomla 4 my website is down with error "Call to undefined method Joomla\CMS\Application\SiteApplication::isSite() ".
I am wondering if there is any chance that our actual template works in Joomla4 & Joomla5 or what is the closest template to get my site online again.

Switch the template in the backend to standard Cassiopeia. 
The new templates no longer work under Joomla 3 and must be reinstalled and rebuilt from Joomla 4 onwards.
If you are still on Joomla 3, switch to the standard template Protestar. 
Then update to Joomla 4 and then to Joomla 5. 
Now you can install the new template and Astroid Framework.

I am on Joomla 4 now but I do not want to have too many changes on my template. Could you please let me know what is the closest template to the Perfect II template from joomla 3.

JP-perfect ist now TP-Perfect but the work is the same for everyone. You have to check the module positions (many more functions than before) and possibly create new ones. Once you have done this, simply assign your modules to the new module positions. The old Widgetkit stuff doesn't work anymore in Joomla 4. Instead, you can use the new and better UIKIT modules. First install the Astroid plugin and then download the template. Please only install the template itself. You can find this in the download zip file under Template /tp_perfect. The downloaded zip file also contains Joomla, but you have already installed it. So only install the template and not the whole package.