Commonwealth Bank was the only one of the big banks to increase its business banking customer satisfaction rating in April, and it has opened up large gaps over its rivals in the large business and medium business segments.According to the latest DBM Business Financial Services Monitor, CBA's overall rating with business customers increased from 7.4 (out of 10) in March to 7.5 in April. ANZ's rating fell from 7.4 to 7.3 over the same period, NAB's rating fell from seven to 6.9 and Westpac's remained steady on 7.4.In the large business segment (businesses with $50 million or more of annual revenue) CBA has a rating of 7.9, compared with 7.3 for NAB, 7.2 for ANZ and 7.2 for Westpac.CBA also leads in the medium and micro business segments.Westpac has the highest satisfaction rating in the small business segment (revenue of $1 million to $5 million a year).Over the 12 months to April, ANZ and CBA have improved their overall ratings, while Westpac has remained steady and NAB has lost ground.