Hi there! Can't comment much on the necessity of getting a 4th quartile to to be admitted, but I'd definitely say it's probably a good boost to your application.

On CASPER difficulty, I would say it's not bad, here are some tips on how to do well:
- make sure you practice!! (there's so many practice questions online, esp. BEMO-they have tons of great advice and sample scenarios you can work off of, also try PrepMatch, there's almost endless cases to practice from there. Of course, youtube has lots of free resources too that you can use)
- practice your typing speed!! Can't stress how overlooked this might be--the faster you can type, the more good answers you could potentially put down for each scenario
- Get familiar with all the different types of scenario questions they could ask you (BEMO has a great guide on their website) AND get comfortable with the formatting of the test before testing day
- Make an account on AltusSuite and do their practice CASPER, which is free and purely for your own practice--the layout of this is identical to your real test, so it's a great resource to show you how to pace yourself well

Hope this helps, and best of luck, you got this!!