No. KDE4 applications work fine in Plasma 5, as they do in any other DE.
Plasma5 even changes KDE4’s settings to make the apps look more integrated.
But as Plasma5 uses different directories, ServiceMenus and Device Actions (e.g. “Download Photos with digiKam”) installed by KDE4 applications will not be seen.
Does Plasma 5 take more/less or about the same resources as Plasma 4? I’m running openSUSE on a 2 core Intel w/4GB of RAM and embedded graphics.
Hard to say, and depends on your settings of course.
It does take a bit more RAM, but work is going on to optimize that, might end up in 5.4 already.
Otherwise there should not be much difference to KDE4 I’d say (with comparable settings).
I have it running on lower spec systems than you and it runs fine, feels even a bit faster than KDE4.
At this stage of the game, is the non-Tumbleweed Plasma 5 relatively stable?
Yes.
But 13.2 only has 5.2.2 at the moment, whereas TW is at 5.3.0.
You can use the additional KDE:Frameworks5 repo though (which is the devel repo for Tumbleweed), which already has 5.3.1. You get stable updates there even before TW gets them, at the day of release. Those packages are then submitted to TW.
http://download.opensuse.org/repositories/KDE:/Frameworks5/openSUSE_13.2/
If you use that, it might probably be a good idea to add KDE:Qt5 as well. Otherwise you might get problems when Qt 5.5 is released, as it will take some time until it is released as update for 13.2 (if it ever will be), and the packages will already need it.
http://download.opensuse.org/repositories/KDE:/Qt5/openSUSE_13.2/
I do also maintain a repo of co-installable Plasma5 packages branched off from KDE:Frameworks5, if you add that too and prefer the packages from there, you can even install Plasma5 while keeping KDE4 as fallback or whatever.
http://download.opensuse.org/repositories/home:/wolfi323:/branches:/KDE:/Frameworks5/openSUSE_13.2/
My repo contains also a lot of KF5 applications (the released ones where available, otherwise development snapshots) that are also co-installable with their KDE4 counterpart.