I'm guessing the differences are:

Internal cable routing for SL4
Lighter weight (is it significant?) SL4
"better" carbon (showing up in weight savings for SL4 and perhaps stiffness?)
a lot of extra cost for SL4 frame.

I wonder if I, the average rider, would actually notice much difference. I can't try them both out at my LBS. I do like the internal cable routing a lot though. Not sure it alone would be worth the money to me

Currently theres 3 roubaix models.

Sl2 external cable model - i had this its a very comfortable century rider.

Sl3 internal cable model - i had this its comfortable but stiffer, still a great century bike.

Sl3 s-works - my current ride, lighter than the sl3, very nice all around bike.

New sl4 - have no idea... Hope it remains as stiff


IMO the sl2 is perfect for people with back illnesses, sl3 for everyone else.
Also, the difference in stifness from sl2 and sl3 is notisable. The sl3 feels like the roubaix had a kid with a tarmac...
Cheers

I have a '12 Roubaix Pro SL3. Love it...simply a great bike. I bought it for the riding position. Handling is neutral...no surprises.
I believe the average rider will feel the stiffness difference. That said, there is likely no or very little difference in speed...especially for the low watt average rider. Generally stiffer bikes feel more responsive and quicker but reality is there isn't much difference in terms of speed. Stiffness is a by product of reduced weight...higher modulus carbon fiber has higher yield strength, less material used = lower weight. Will you notice the weight difference? I would say not...probably the difference in filling your water bottle.

Even if you local bike shop doesn't have both models, it is worth a drive to ride the different models since there is a major difference in price as you noted. Also, groupset and wheelset matters in terms of overall feel and performance...perhaps as much as the frame.
No losers. Buy the one your budget will support.

I bought a Roubaix SL3 Expert a couple months ago and *love* it. I'm definitely an average rider—a woman in my 40s working on getting out of the athena category.

My first bike was a Schwinn KOM 10 and had it for 20+ years until last summer when I bought a Cannondale Quick. I re-discovered my love of cycling and rode the heck out of that bike. I had never ridden a road bike until testing them this summer. I tested various "endurance" style bikes, but tried a Super Six for fun. I loved how that felt, but was talked off the ledge, so to speak.

When we went to our LBS that carried Specialized, I rode the Ruby and Roubaix. I made a mistake by riding the Roubaix SL3 Expert . I immediately loved how it rode and handled and it reminded me of the Super Six, but with the endurance geometry. It felt more "fun." I continued testing bikes, but nothing could top the SL3 for me for fit and fun factor.

So, this very average rider felt a difference. It was a splurge, but for me it was worth it. I'm riding around 100 miles a week and am thrilled with my dream bike. I'd get it all over again.

Check your calendar... because C4L and I rarely agree , but I think he's spot on. I've done long test rides on several of them but haven't owned any. I would make some changes for fit, but the stock bikes ride really great.

I think the handling was the biggest thing I noticed on a SL3 and I've read that is ever sharper with the SL4.

Its been a while Colin and I forgot what we were arguing about before...lol.
I will say it another way. We generally agree about things and sometimes I just don't say so.
Cheers.

I love my Roubaix SL2. The way it smooths out even the roughest pavement while still being responsive and efficient is amazing.
I wish Specialized would make a Roubaix with a rack mount for commuting.

Some of this is surely the placebo effect. I think it would be hard to feel the difference between an SL2 and SL3 unless you were Cancellera. But I digress ;-)