Base on my experience, it should go like this. It's not 100% accurate for between 1~34 but I hope it helps.

< 30 -> Grade 1
30~44 -> Grade 2
45~55 -> Grade 3
70+ -> Grade 4
Levels above indicate the requirement to get 100% of that grade. Higher level of that range have chance to get next higher grade.

I can confirm that lvl44 gear can still give grade 2 and mythril tier accessories (Starting 35) gives a grade 2 (100%) or 3.