Can anyone recommend the right size and pitch prop for a 2004 (twin step) 380 Avenger with 525 EFIs in it?

30 bravo

I run 30" hydromotive 5 blades on my 2005 with 525's and they work great.

What ever you go with go with a 5 blade. the 4 blades suck bad on these boats. I run a 29 5 blade

What is the reason why the five blades are so much better?

Check out the Mercruiser site at http://www.mercuryracing.com/propellers/maximus.php this will tell you the difference between the props. The Maximus props are very good for your application. Also the Hydromotive props work good you can check them out at http://www.hydromotive.com/ both these sites offer tech advise on the difference between the 4 and 5 blade props. The five blade are generally more money but perform better at all rpm ranges. The 29p to 30p sounds correct as the 38 with 502's 425 HP turn 28p props. There a great boat dealership in your area called The Boat Connection the owner is a world champion race boat driver and actually owns a PowerQuest boat he is very knowledgeable about props the number is 561-686-0002 ask for Howard.

The 38' Powerquest with the 525's will blow out most four blades easily. I had a set of 4 - blades custom set up for the boat and even they would blow out under hard acceleration out of the hole. The five blades do not blow out and you get better performance through the entire operating range. I tested both the maximus and the hydromotive 5 blades and the hydromotive worked the best. If you call hydromotive they will set you up with the correct prop for that boat. It is a different boat with the 5 blades.