You may contact sales@carbide3d.com to get their opinion, but here’s mine for what it’s worth.
I used a SO3 for a few years and recently upgraded to a Pro, the SO4 sits somewhere in-between. The linear rails help minimize the deflection of the router under cutting load (especially the front/back deflection, around the X axis), so they matter most when using aggressive feeds and speeds to maximize productivity, or cutting hard material (aluminium, hard wood).
@LiamN has done a fantastic job of explaining/measuring that in this thread, it’s a long read but a very interesting one if you want to have an idea about the actual deflection values, there’s a part of the post specifically about v-wheels.
Linear rails are most useful for folks who cut lots of metal OR need to maximize material removal rate in other material to boost their productivity / minimize cutting times.
I see you intend to machine exclusively acrylic, with a focus on accuracy versus speed, so I would tend to think that a SO4 would work great for that purpose. I don’t have any accuracy figures for SO4 vs Pro in acrylic at reasonable feeds and speeds, but my intuition is that since you will be cutting complex 3D designs, you will use a roughing pass to get most of the material out of the way (and accuracy is not a concern there), then follow-up with finishing passes to mill the fine/final curves, which by definition put very little load on the tool/machine, so there will be virtually no deflection, or at least none that the SO4’s vwheels can’t handle.
As usual, the level of future-proofing you want to do comes in the picture too, if you think you will have other usecases down the line. But for the scenario you mention, I would go for a SO4.