My opinion on it is mostly from my experience in tabletop 5e. Level 5-8 plays very well in a lot of environments/encounters, while levels 1-4 has always felt more underpowered than it should be. +3 proficiency makes a strong impact in 5e.

Originally Posted by 1varangian
No increase at all until level 9? You gotta have some progress or it gets boring. I'd be more generous, like...

Level 1: +4, Level 4: +5, Level 8: +6, Level 12: +7, Level 16: +8, Level 20: +9
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Condensed for ease of reading - Dragonsnooz

Moving the first increase from level 5 to 4 would ease the huge power spike on 5 a bit. And it feels right having a final increase on the maximum level. Maybe it would be horrible in the end.. but more progression and being better at trained skills feels like a good idea.
The classes have other ways to scale up, for example Fire Bolt gets and addition d10 damage & Fighter gets extra attack. Level 5 is a significant power bump for the party, without the proficiency change.

Also keep in mind I want to leave room for varying difficulties, so normal could be a static +1 to all proficiency, easy could be a static +3 to all proficiency levels. (Hard would be +0). If the game were to go one difficulty, I'd consider an expedited proficiency scaling, but I don't see a reason to go past +6. +6 makes a huge impact on the game.

Something like levels 1-5: +3, levels 6-10: +4, levels 11-XX: +5 could definitely work for the game. But as I've said before +3 proficiency feels really good at levels 5-8. It makes more sense to me to give levels 1-4 a bump, and keep levels 5-8 where they are in rules-as-written.