OK. I work at Advance and looked this up. The reason they are recommending Iridium is the benefits it offers. Iridium will last the longest BUT the OE plug is Platinum. Motorcraft's plug recommendation is #SP498 and cost $4.69 each. There is a nickel plug also #SP412 at $3.49 each but it's below recommended specs. It will work fine but will wear much faster than platinum or iridium and if you have never changed plugs in a 4.0 then woohoo!
Next time you go in ask for the Commercial Pro. They will have the most knowledge and will give you answers and good recommendations for parts. I'm that person at my store and have been in parts for 25 years.
The reason most parts stores offer parts that may be more expensive is the OE companies send their parts info to the catalog programmers many times per year to update what they want you to sell. Motorcraft probably has updated your plug recommendation a few times since 03.
Another good idea is go to the branded web sites and see what that brand is wanting you to use. Get your own part number then go get the part. For Ford go to
http://www.fordparts.com . They will show you the latest update on what to use in your truck.
Hope this helps.