Using a DPP44 switch or buying a second one is pointless and a waste of money unless your using a Dish 1000 or actually pulling programming from 4 satellite locations.
A Dish 500 or Super Dish with 2 DP34 switches will allow him to run 6 locations just fine. You need to speak to a local retailer and see what is being installed in your area.
If you are in a Dish 1000 market a second DP44 (Dish should provide the first one because you want 4 TV’s) and either buy another Dual Tuner Receiver or two individual receivers. Dual Tuner would be cheaper monthly fee.
Since Dish under any plan only provides installation of programming on up to 4 TV's, whether 4 individual receivers or mixed with Dual Tuners the mirroring or installation of additional receivers is custom labor and subject to hourly rate that the tech charges and what ever fees Dish charges for the additional receivers.
Now since you had to ask “What's involved with installing your own receivers? Is it easy to do?” I would suggest working out a deal on the labor with the installer when he shows up or as Wobbie suggested going to a local retailer and trying to work something out with them.
If you can get a 322 Dual Tuner Receiver from RS for $149 go for it, but make sure there isn’t a catch like activating it through them on a New account only or something like that.
Near as I can figure your monthly cost will vary from $42.98 to $62.98 plus tax as long as there not DVR or HD Receivers. The 5th and 6th TV will have the additional programming fees, but if you use a 322 for them and as long as it’s plugged in to a phone line then you should only have to pay one $5.00 fee for the 5th and 6th TV's.
Even under the DHA, whether you own your receiver or lease you will pay a $5 fee for every tuner, exception is the dual tuners if there plugged into a phone line. And according to the DHA the maximum number of TV's is 4. That’s why Dish wants the extra $425.
If I'm missing something or stated something incorrectly I'm sorry as I was in a hurry when making this post.
Dish DHA Agreement
http://www.dishnetwork.com/content/getDish/disclaimers/dishPVRplans/index.shtml
DHPP Agreement
http://www.dishnetwork.com/content/customercare/warranty/terms/index.shtml
Residential Agreement
http://www.dishnetwork.com/content/aboutus/rca/