Can't replicate this issue. Which extensions are you trying to install and what errors are you getting?

The Extension was the Date & Time Extension obtained via the JED and attempted to install via Package Upload.

Can you post a link to the extension on JED?

Here is the link to the Extension:

https://extensions.joomla.org/extension/display-date-time/

Call to undefined method Joomla\CMS\Application\AdministratorApplication::isAdmin()

This extension is not compatible with 4.0.

OK, thank you very much for noting that for us. We will check further Extensions closer hence. Thank you again.

Please close this

Please note that Joomla 4.0 isn't in Beta yet. It's still Alpha.
And obviously neither Alpha nor Beta should be used in production.

Thanks Bakual:
Was using "Beta 1" on a prototype testing site, not for production. Appreciate the advice, but we are/were well aware of the version used and the cautions about using on a production site.

@Bakual If beta is not yet out, what is the nightly build Beta 1 currently available on Devleoper network page https://developer.joomla.org/nightly-builds.html ?

Joomla! 4.0
These packages were built based on commit fb71685
These packages were last built: Tuesday, 17 December 2019 03:00:45 UTC

Joomla_4.0.0-beta1-dev-Development-Full_Package.zip
Joomla_4.0.0-beta1-dev-Development-Update_Package.zip

beta1-dev means it's still in the development process for the upcoming first beta version. It's not a released beta yet (thus still in alpha version). After first beta (4.0.0-beta1) is released, the version in the nightlies will become beta2-dev until beta2 is released. And so on.