It’s depends on gallery width and thumbnail size which you set in gallery options.
The gap between thumbs is 10px and gap on the sides are 20px each. So, if you need 4 columns with 150px width of thumbnail:
20 (gap) + 150 (thumb_1) + 10 (gap) + 150 (thumb_2) +10 (gap) + 150 (thumb_3) + 10 (gap) + 150 (thumb_4) + 20 (gap) = 670.
You should set gallery width to 670px.
In other way if you have 600px of Post content width (gallery will be 600px too by default). To know thumbnail width you need:
600 (gallery width) – 20 (gap) – 10 (gap) – 10 (gap) – 10 (gap) – 20 (gap) = 530,
530 / 4 (columns) = 132.5
You should set thumbnail width to 132px.