Questions about Bilsteins for the rear.

I'm eventually (as money allows) going to be building up my own 4.5" lift. Recently a pair of rear-only Bilsteins came up for sale, brand new, for $100 in my area. The owner says they're for a 4" lift. I had originally planned to buy 5100's front and rear.
These are yellow with the blue boots (whatever series that is), and they have the bar pins. Am i going to be missing anything if i pick these up for the rear and go with 5100's for the front? Is the difference in valving, or flex, or what? Is that a good price?

I'm a little split on what to do, so any thoughts are appreciated. My Jeep is a second car and it has always been a "budget build", but i don't mind spending more if i'm really getting more...

Yellow and blue are the 4600 series. Wasn't aware they had those for 4" lift. Make sure by asking the extended lengths should be around 24"s. They sell new for $66 a peice so its not that good of a deal.

I'd keep saving for the 5100's.

Yeah, i was a little surprised that they were the yellow ones...thought with the lift size they'd be 5100's for sure. Thanks for the info, sounds like i'll pass for now!

Hold out for the 5100s, like XJWOLF said. I'm very, very, very glad I listened to everyones' advice and waited a little longer to buy them rather than some ProComp low-end crap ($30 a shock, I think they were ES3000's?). The only thing that would improve my ride quality would be drop brackets or long arms.

I am a little bummed I didn't save a bit longer for 7100s, though. I've made peace with my lift height and tire size and have no interest in going higher (4.5" on 31's), so I should've bought a "lifetime" shock.