I've got three hunchies over 100k past mastery. Here is my advice regarding them:
If you want to go for an ac20 or a gaussback, get the HBK-4H and ignore the 4G. The 4H lets you take up to five medium lasers in addition to your big ballistic weapon, which adds a lot of additional punch, and leaves you with options besides the main gun if it goes out. The 4G only has three energy slots, which it trades for two additional ballistic hardpoints. This means that the only thing it can do that the 4H can't is 2x AC2 or 2x UAC5, neither of which are considered very good loadouts.
The 4SP is a very solid mech, and you can't really go wrong with it when you're learning how to run mediums. It's fairly tough, has pretty good close range punch, and having four mlas on the arm crosshair is a huge help against lights.
The 4P was the best medium for those with steady aim and good trigger discipline up until the last patch for its ability to put 40-80 damage on target and then get out at a decent speed. You can't use all the lasers that it comes with anymore, so there is no real reason to buy it now.
The 4J is a bit of an oddball- on paper it looks better than the 4SP, with more missile tubes and one more laser hardpoint, but unlike the 4SP, it has a huge hunch that leaves it vulnerable to losing 80% of its firepower to one shot from a heavy or assault. It's not a bad mech, but given that its hardpoints are so similar to the 4SP, it's hard to justify using it when there is an option available that doesn't leave the pilot so open to being disarmed.
No matter what you wind up picking, be prepared to get killed in 2-3 shots by most heavier mechs. Most hunchies will go about 80-90kph with a maxed engine, speed tweak depending, so you're a fairly large target that isn't very fast, and most bigger mechs regard you as free cbills. The Cent is more durable and faster than the hunch despite being larger, mostly thanks to its broken hitboxes. I don't drive any cents so I will let someone else break them down for you, but in general the HBK has more firepower and is easier to hit with, as well as having more options for loadout diversity, but the Cent survives longer and goes faster.
Edited by aniviron, 20 July 2013 - 07:14 PM.