My question is, in general, does using a combination of optimization plugins actually result in a net speed increase for sites?
It depends, they might make it faster, they might make it slower, it depends on so many variables, how good your hosting is, how busy your site is, what exactly the plugins are optimising and how they're configured, etc.
The only way to answer this is to just do it and take timings to see if it's faster/slower. The only other general advice to give is that optimisation plugins don't always improve performance, and you can't just keep adding them.
It would be great if I could give you a simple directive on this that always held true for all people. Just do it, and see the results.
In other words, could installing say, 4 optimization plugins actually outweigh the performance tax of adding 4 additional plugins?
I'm not sure what this performance tax is. Plugins are just PHP files, it's what the plugin does that incurs the cost. 1 plugin that does a lot will have more of an impact than 1000 Hello Dolly plugins.
A lot of people say that you shouldn't add lots of plugins, and get the impression it's the number of plugins that slows the sites down, that there's just something intrinsically slow about plugins. That's not true. It's the amount of stuff your site does that slows it down, and plugins are a great way to do more stuff.
I know that this is may depend on the optimization issues themselves, I just want to know if this is ever advisable in general.
Extremely dependent. To be honest, you'd be better making the changes to your theme rather than installing bits of software that promise to magically fix things