Yeah, the Nforce 4 is a chipset, not a motherboard.
Chipsets - Nforce 4, Nforce 4 Ultra, Nforce 4 SLI, etc
Motherboards - ASUS A8N-SLI, DFI NF4 Ultra-D, etc
A motherboard uses a chipset. For example, an ASUS A8N-SLI uses the Nforce 4 SLI chipset, which allows for two GPU's to be run at the same time, which boosts performance.
Now if you are asking whether or not an Nforce 4 SLI board will out perform an Nforce 4 Ultra board, they will run identically unless the NF4 SLI is given aother GPU to enhance performance.
Note - I'm talking about the Nforce 4 SLI vs. Nforce 4 Ultra, not the plain NF4.