I'm not aware of any 5.5" spindles by CST, unless it's something new...unless you are talking about the CST spindles with a 2" spacer
With the CST, you can keep your factory front shocks, and your 3" front level, which will give you 6.5" lift up front. (you will maintain your 3" lift, and the CST spindle will lower your wheel hubs 3.5"). The procomp achieves about the same amount of lift, except using different different parts and methods, and it will reduce your clearance up front (not really a big deal since you don't offroad). I'm a fan of the CST look from the front, because there isn't a big bracket sitting under there...just my opinion, and of course, because I have them
Procomp 6" Stage 1 lifts drop your lower arms with brackets, and uses a spacer to take up the extra space between the top of your strut and the frame, as well as adding a little bit of lift with it. I can't remember for sure, but I think the PC 6" stage 1 lifts are around $2000-2500 or so, and come complete with everything you need, front and rear.
If you go the CST route (~$850 for the CST spindles), all you'll need to do is get some lift and shocks in back. The cheapest route would be to order some blocks, (I'd go with a 4" block for 6.5" lift up front) with ubolts, shouldn't set you back more than $130 or so. As for shocks, you can get a new pair for around $200 or so. You'll need a carrier bearing drop bracket, to get your drive shaft angle corrected, that's only $80. So, you're talking around $1300 +/- to get a 6.5" lift, and keeping the 3" spacers that you already paid for. (which is exactly what I did). You'll also pay a lot less in labor to install the CST spindles vs. the PC, unless you do it on your own, which is free. Seems like a PC lift might take a full day, at a shop, whereas a CST spindle, blocks and shocks in the rear might be about a 5-6 hour job.
Back on topic... Pro comp 4" vs. 6" lift. If those are the only 2 lifts you are considering, the procomp 4" lift would be ridiculous to install on a 2wd. 4x4 guys are buying that kit so they don't have to cut their cross members, like the PC 6", and any other 6" lift requires. As far as I know, there is no cutting of the cross members for a PC 6" lift to be installed on a 2wd truck. So, Pro Comp 6" wins for you, because the 4" would be a helluva waste of money, and you'd be sitting 1" higher than you are now
But, I'd suggest looking into the CST spindles anyway..the things are badass, built like a tank
There are also many different configurations you can use, with the CST's. When I bought mine, I ordered some Bilstein 5100's, because I was only wanting around 5.5" of lift or so. Well, the 5100's were on back order, so I canceled that plan, and just re-used my 3" level kit spacers...did more to it later, to get more lift, but that's another topic. Guys are running all kinds of configs with CST: CST + 5100's, CST + level kit spacers, CST + coilovers, CST + OME, etc. Check out coachbuilder1's truck, he has CST +OME, and is sitting at around 7-7.5" lift or so. He says it rides great. That will be my next setup, when $500 falls out of the sky
1 caveat to the CST spindles, which I didn't know until after they were installed. They widen your front track about 1" on each side. Some people don't mind it, but I went ahead and put wheel spacers in back to even it out. This is also something to keep in mind if you want to go with low offset wheels. I have Cranks with 0 offset and 35's (325/65/18). I had to trim my body mount and front bumper to make it all fit when I was at 6.5" lift. I eventually went up to ~8" lift, and had I done it that way first, I might not have had to cut on anything.
Here, check this out:
CST 2007-2012 Toyota Tundra 3.5" Lift Spindle
Here is my truck with CST's, and total lift of around 8" up front. Without the extra hardware that comes with those bracket lifts (drop brackets), I think it makes the truck look a little taller when viewing from the front.