How can I use /setblock in 1.12 to make a blue banner?
This answer is for 1.12 and older versions.
Your attempt using Base:4 is the closest to the correct way. However, this is attempting to set a data value. This only sets the direction of the standing banner, not the background colour.
You need to move the Base:4 to the area where NBT tags are stored, in the {}s after the data value and old block handling.
/setblock ~ ~ ~ minecraft:standing_banner 0 replace {Base:0}
Here's a refresher of the possible values for the banner, for yours and the viewers' conveniences.
- 0: Black
- 1: Red
- 2: Green
- 3: Brown
- 4: Blue
- 5: Purple
- 6: Cyan
- 7: Light Gray
- 8: Gray
- 9: Pink
- 10: Lime
- 11: Yellow
- 12: Light Blue
- 13: Magenta
- 14: Orange
- 15: White