Safari 8.0 on Yosemite doesn't kill closed tab processes, hogs memory
"Web Content Process" = "Tab" is a wrong assumption. A Web Content process can hold more than one tab. Depending on the implementation details of Safari, it might be possible that after closing the last Tab related to a particular Web Content Process, it does not automatically mean to close that Web Content Process. It might be feasible for Safari to keep that Process idle in the background ready to take a new tab when needed. I think Mavericks Safari behaves the same. 

On your experience of "slower and slower": It might be that 4GB of RAM are insufficient for your normal workload. As a result, virtual memory is used. That means your HDD (or SSD) is used as an extension for your RAM, when more memory is needed to drive all open processes. Usage of HDD means a slowdown. An SSD is better in that respect.

I use Safari 7 on Mavericks, I will not test now what happens when closing a big enough number of tabs. But let me describe the normal situation on my MBP: At the moment, my Safari has 4 windows open, each contains 10 to 40 tabs, in sum ~100 tabs opened. In Activity Monitor, I can see 20 Web Content Processes, each 9 to 12 threads. In addition, I have 13 QTKitserver Web Content Processes, each 2 threads.
All web content processes have a 3-digit PID, which means they were started at the beginning after my last reboot, which is 3 days ago. So the Web Content Processes are reused by Safari. If they were new processes, they would have gotten a 4 or 5 digit PID. Though, I admit I did not open and close that many tabs since the last reboot. I have 8GB of RAM and an SSD on a late 2009 MBP.

You can read at some places that Yosemite needs a little more RAM than Mavericks. Maybe that's enough to slow down your 4GB system in comparison to Mavericks. Though it should not perform much worse than Mavericks. By the way, a nice read on memory management for 10.9 can be found here.

Bottom line: Consider installing 8GB RAM. It's dirt cheap for a non-Retina MBP. 

And you can do it yourself. Just figure out before which RAM-Modules are for your MBP Model:
https://support.apple.com/kb/HT201165?viewlocale=en_US#1.

It seems that even 16GB are possible for even older MBPs if one is lucky. Beware, this 16GB topic needs more research: https://forums.macrumors.com/showthread.php?t=1511196

But I do not know if that is needed in your case. In my humble opinion, 8GB are fine.