A 4' level is more than sufficient to check how level your tank is. Should be plenty long enough to set across the tank width-wise, length-wise, and across both diagonals (left-front corner to right-rear corner, and right-front to left-rear).
If you are only seeing a difference from back to front, odds are your floor has a little sag to it as you move further away from the wall, causing the tank to lean in that 1/16". If it were me, I wouldn't worry about it.
If you are worried about it though, you could shim the feet in the front 1/16" and then any feet on the sides that are no longer touching the floor will need slimmer shims (less than 1/16) as well. (E.g. No foot should have an air gap beneath it, each foot should have support beneath it.)
Or, if it'll sooth your nerves a bit, you could shoot Red Sea customer support an email and ask them if being off front-to-back by 1/16 is something you should be concerned about.
As far as a twisted or 'racked' stand top goes, you can check this by getting a level or other item that is known to be a perfectly straight, non-flexing edge and moving it across the top in different directions to make sure there are no dips or rises in the top. E.g. it's perfectly flat.
At the end of the day though, IMHO, people let emotion feed their worries rather than letting logic keep them in check.
Red Sea knows ordinary, non-engineering folks are going to buy and set up their aquariums. It's a safe bet, they've engineered the joints and construction of the tank with a safety factor to account for the 1,000s of set-ups out there that won't be absolutely 100% perfectly level.
Likewise, they've designed and QC'd the stand and its top to be within a specific flatness tolerance to prevent stress on the tank.
Unless your top looks to be noticeably out-of-flatness, or your are setting it up on very uneven flooring that will allow the stand to essentially twist as weight is applied to it, it will probably be just fine.
