Took the time to make a chart for everyone to use. Keep in mind this is always subject to change and I'll do my best to make changes as they come up and update the post.

As of December 13th Patch 1.12(Vita) and 1.03(Ps3)

*Notable Finish = top 25%
**Need more data for an accurate point requirement for G4.

This is a chart that is accurate for players before demotion to the last rank, keep in mind that the point requirements may change for promotion after you were recently demoted. This requires more testing but the chart provided should give you a good indication of the requirements to maintain grade or to promote to the next one.