How large of a batch do you plan to make, and how big/small of stir bars will you use?
I mostly use [for e-juice] 100ml and 250ml beakers and up to 1" stir bars…sometimes I may use a 500ml beaker and 1.5" stir bars. The old Mag-Mixer I use has a rod magnet that is the same size as my forefinger [index/pointer finger], so being a continuous magnet it will spin anything from a .25" bar to over 3"
Almost positive all of my mag-stirrers have the commercial rod magnets.
Having two magnets, you will need to decide the average length stir bar you will use, then place your magnets accordingly for best results. I am not sure how 4 magnets would work in two configurations, but it does sound interesting, but not sure which direction each one should be set. I would stay with only two.
Have you watched @Code-Vape video on youTube? He says his works very well [HDD magnets , rheostat]
One other note for e-juice use… I NEVER worry about a vortex when mixing, and generally only have one at the end of the cycle, but at that point my temp is high enough that I reduce the rpm’s.
Lab testing chemicals, yeast starters for beer/mead, etc is a different mix situation than e-juice. I want a zillion tiny bubbles blending as the temp slowly rises for my e-juice, not a tornadic vortex. I do match my stir bar to my volume/rpm’s for a vortex when I make my organic pure stevia powder stone, otherwise it would take all day to get it blended, but I never stress my juice that way.