Your 760 is shown in the Device manager, and would not if no drivers were installed for it. If there were ni drivers it would say standard VGA graphics adapter. If you don't set Windows Update to let me decide what to install, it will install a graphics adapter, which is OK, bit it will install a graphics driver for the card. The purpose of that tutorial is to get as clean an install as possible, so, you wouldn't want Windows to install a graphics driver and you install one too. Having said that, it is done all the time and some people have trouble and some don't. That tutorial was written for those who do. If you prefer, just leave it like it is unless it is causing you problems. My philosophy is, if it ain't broke, don't fix it. If it runs well like that, just go with it.
I don't think 4 GPUs could possible be bad unless you were the most unlucky person in the world. And even then I would doubt it. So, if we assume the GPU is not the problem, that leaves the Motherboard or the PSU as the problem. If drivers are properly installed, That would be the only 2 other things that could affect it. And, there are no tests for them. You just have to guess and have a 50/50 chance of being right.