I’m looking at using a combination of Futaba (S3004) and Hitec (HS-485HB) servos, running at 4.8v. Does it matter if they’re not all the same manufacturer?

No it doesnt, most of my airplanes have more than one servos brand and they work fine.
however, I have one hitec hi torque servo maybe HS 375 and it draines the Rx battery much faster than if i put two simple servos.

You can mix and match servos on a plane with no problems. The only place you'll want to make sure to use the same servos is when you have multiple servos on a control surface, such as 2 servos for ailerons, rudder, or elevators. You'll want the same type of servos in this case to ensure that both servos move the same with the same speed. Different brands of servos could be enough off each other to make a difference.

Ken

What about needing servos that have a common "connector"? Can Futaba, JR, Hitec, etc. all be connected to the Rx without needing some kind of adapters? Are adapter extensions necessary that have a Futaba female end and a JR male end (or vise versa)?

You're in luck, Try this site: http://www.cermark.com/categories/Connectors/

JR plugs into Futaba.

To plug Futaba into JR just shave the index tab off the connector.

As BarracudaHockey states, Futaba needs to have the tab clipped off in order to plug into other radios. But all of the others will plug directly into each other without any issues. Airtronics, JR, and Hitec all use the same plugs and are interchangable as well as being able to plug into Futaba receiver. The only exception is the older Airtronics connectors which can be identified by the plugs being black and having 3 ridges on the plug.

Ken</p>