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.