I wanted to ask the developers whether 4.3.6 will be the final point version of 4.3 I would like to upgrade our portal with some of the latest bug fixes, but hold off any new functionality from 4.4. Is it now a good time to upgrade?

I can't resist answering to this question.

Yes, 4.3.6 will be (or rather: has been) the final point version of 4.3. After being released on 9 January 2008, it has been superceded by 4.4.0 on 25 Jan 2008.
And yes, if you're on any version 4.x or earlier, the good time to upgrade started more than 10 years ago ;)

That being said: Whatever software you're running, specifically server-side: Prepare for upgrades, if only for a security fix. You can never rely on anything to be final - and you should stay reasonably close to the latest release. If you increase the upgrade frequency, the changes from version to version will be smaller and smaller, thus the upgrade efforts will be lower per step. Also, it will neatly show you if you have any invasive customization that requires lots of attention (hint: Try not to have those).

In case you've mistyped 4 for 7, and meant to ask about 7.3.5 GA6: Please clarify. I'm not aware of plans to release another 7.3 CE series, rather 7.4 is in the making. But that's not a guarantee that it won't happen. Nevertheless: I wouldn't expect new features in a version 7.3.6, but only (or largely) bugfixes if it is released.

Is now a good time to upgrade? Yes. Always.