I'd say its safe to bet it would be 10% faster. Since the 4870 is 20% faster with same core and GDDR5.
Hmm...I see your point, but you have to look at the actual memory clock speeds, not just the type of memory. This card with the GDDR4 is only 2000MHz. The GDDR3 version's speed is 1800MHz. The GDDR5 is clocked at 3600MHz. So, going by memory clock speeds only, an increase of about 10% is theoretically possible. However, keep in mind that the 4870's core speed is exactly 20% more than the 4850's (750MHz vs. 625MHz.) So really, for the most part, the GDDR5 memory does nothing for the 4870. I guess maybe its higher latencies offset its increase in raw clock speed, maybe?
To sum it all up, I'd say that the GDDR4 would do very little for the 4850. Maybe 5% extra performance at the very most. It might lower the power consumption as well, since I seem to remember reading that GDDR4 eats up less power then GDDR3.