Marmoset Toolbag 4 is a new-comer to the field of texturing - Toolbag 3 was primarily focused on real-time rendering, and as an alternative baking tool.
PROS
Solid all-in-one tool, that allows to visualize your texturing in the final render setup.
Strong baking tools.
Powerful real-time rendering.
CONS
UI can be confusing, as there is a lot going on.
Limited painting tools, since they are a very new addition.
Limited resource library and rudimentary custom resource setup.
Learning Resources
Tutorials, documentation and art breakdowns:
https://marmoset.co/resources/
Official group:
https://www.facebook.com/groups/ToolbagUserGroup
Discord:
https://discord.com/invite/5YsseU4Cx2