You will not notice any lag in performance. The way that multi-tasking works in iOS 4 is that when you run out of ram, the oldest application is quit and it's memory is released. There is no virtual memory on any iOS device. Virtual memory (swap file) is what slows a "normal" computer down when it is running low on memory.
What you would notice is that the iPhone 4 is able to keep more applications open, before they are forced to quit.