Elixer full stop. Don't even hesitate. Beulah made the rod and line with each other in mind.
Either the 250 or the 275. Could even go as high as the 300 depending on your casting style, but if you want those extra grains for bigger flies and heavier tips, then I'd go with the tonic instead.
Given that it's such a light rod which you might want to fish single handed, you can always go a shade on the lighter side and start with the 250 Elixer and go with the integrated head & line, but if you are just getting used to scandi heads with a switch and often fall back to water loaded casts, then go with the 275.
If you are well experienced with touch and go casting on a short rod, then I'd look at the AFS. The longer head will be more challenging but the longer taper will help you deliver small flies much more delicately than the Elixer with the standard 10ft poly tip. Trouble is that the lightest AFS really is more suited for a 5/6 rather than a 4/5 switch. In the short term, you could always use a 9wt floating line and cut the front 10ft of the line, then put a loop on and attach your sink tips, or don't cut it and just choke up on the head so that you are not using much more than 30ft to load the rod.
It all depends on what you intend to catch and how you want to catch them.
If I wanted to use spey casts to deliver poly tips and flies up to say a #6 weighted wooly bugger or foam hopper, then I'd go with the 250 or 275 Elixer and the full compliment of poly tips including the floating one.
If you want to use skagit style water loaded casts to deliver streamers and weighted things on the end of T-8 then look at the Tonic by Beulah, or any of the skagit shorts in ~275 or 300grains
I assume you've looked at the manufacturer's recommendations:
http://www.beulahflyrods.com/products/switch-rod-series/classic-switch-rods/4-5wt/