Why does Harbinger have only 4 claws?

Indeed, Harbinger design is different form the others reapers.

The Reaper known as Harbinger appears to be of a unique subtype; Alliance intelligence identifies it as being the largest Reaper in the armada, and its design differs notably from the Sovereign-class subtype, having only four main legs and multiple glowing "eyes". In addition, Harbinger's cannons appear to be capable of a much higher rate of fire than those of any other Reaper.
- Reaper article on masseffect.wikia

But why? (Spoiler from Mass Effect 3)

During its encounter with Shepard, Leviathan mentions that Harbinger was the first Reaper created, and that it was formed in the image of the Leviathans. Through Harbinger, the Catalyst directs the Reapers to harvest the galaxy of advanced life every 50,000 years. Each harvest ends with the birth of a new Reaper made in Harbinger's image.
- Harbinger article on masseffect.wikia

Harbinger was constucted from the genetic "goo" of the Leviathans, who were the most advanced race to ever inhabit the galaxy. Meaning that all that advanced tech went into Harbinger's make-up. So that is the anwser to Harbinger's weapons being much faster in terms of fire rate. As for the claws, every reaper is different in some way. I think Bioware wanted to make Harbinger more noticable because he was the main reaper of the storyline. With Harbinger's eyes you come back to the Leviathans who have six eyes in total and if you look hard enough you will notice the same with Harbinger