My synapse currently has fulcrum racing 4's. If my research is correct, they were stock wheels for a specialized roubaix. I can't find much info about them at all, so if anyone knows much I would appreciate it.

I'm considering buying a set of ultegra wh-6800 wheels for it, only if it would be a noticeable upgrade over what I have.

Opinions please!

Lighter and stiffer usually equals better performance. Compare reviews for what you have versus what you want. Then consider what your wallet can handle. I personally prefer upper mid level Mavic wheels.

If the 4's (special wheeler made just for specialized), they are between fulcrum 3 & 5, then the ultegra wheels are probably not a weight upgrade at all.

Hubs and over all build quality MIGHT be a tad better on the ultegra, and they MIGHT last longer. Maybe...

The ultegras are probably a bit more narrow, which in 2017, is not the direction most people want to go. (Most new fulcrum racing wheels are 17mm internal...ultegra is 15).

I've tried numerous times and can't find any info on the 4's that I have.

I also found a solid deal on mavic ksryium elites. How do they compare to the other two wheels mentioned?

Try Shimano RS81. Carbon wrapped, so they're light and stiff.. little better than the Ultegra.
I have a set of Mavic Ksyrium, they're decent wheels. Not the lightest or best rolling I've used, but they're a good wheel. As long as you find them at a good price.

I'm considering a set of the Mavic Pro's... Mavic's seem like a decent wheelset.

My brother had some RS80 and loved them, so I've looked at these also. It looks like it might be hard to find them in the price range that I'm comfortable staying in, but I am interested.

Not sure what your budget is, but I saw them online today for $499 I think. Which makes me sad thinking about what I paid, lol. The C24's that is.

I saw those for that price on nashbar. Figured if I waited for a 25% off day, I'd get them for $375.
Technically, I could afford it, but I'm not sure if it's a justifiable way to drop $400. I'd like to spend less if possible.
I've never changed rims before, so knowing whether it's "worth" the money to me is tough.

Mavic sucks at everything now imo, proprietary crap thats behind everyone. Good luck getting help if you have a problem or broken spoke

At the price range you are looking, keep the Fulcrums. They are great wheels, and i doubt the Shimano's would be an upgrade at that level. I also don't like Mavic, they seem overpriced for a proprietary system that doesn't do anything better than other options. If you want to upgrade to a nice wheelset, just save up a bit and make it worth it.

Fulcrum racing 4s are 1725gm a pair.
Ultegra wh 6800s are 1640gm a pair; not enough difference to justify an upgrade.

I have the aforementioned Shimano RS81 C24s, they are a little over 1500gm a pair, and are great if you are set on upgrading.

If you can find a new set of the old model Dura Ace WH 9000 C24s for under $700, those are under 1400gms and you would feel the difference from your current wheels.

I"m assuming you already know that, unless you are racing or otherwise performing at a very high level, paying hundreds of dollars (or more) for upgrading to wheels lighter than you current ones is just for fun/vanity; if you want to get faster or go further, you spend your money and effots on (first) optimizing bike fit and then on a disciplined training program, with or without a coach.

Thank you for the info. Yes, I completely know and understand your last paragraph. I don't race and I won't train, which is probably a large part of the reason I cannot justify to myself spending more than 3-400 bucks. It's completely for fun/vanity as you mentioned, but also curiosity.

The only other info I have been able to find on my wheels had them listed at 1816g. I figured losing a half pound off my wheels by switching to something like the RS80s would be noticeable. Is it not enough? Would I have to look at something closer to 1400g to notice?

I'd guess that losing half a pound of rotating mass would noticeable, but it's just a guess.

On a separate note, getting your bike fitted for some aero bars is a huge bang for your buck. I found that my avg speed over a 50 mile ride went up by 1-2 mph.