National Australia Bank has led growth in retail customer acquisition among the big banks by a wide margin over the past four years, according to new research.Roy Morgan Research looked at the increase in personal customer numbers from July 2011 to July this year. The Big Four added a total of 1.04 million customers.NAB's customer numbers grew by 15.7 per cent over that period, followed by Commonwealth Bank (5.2 per cent, including Bankwest), ANZ (2.7 per cent) and Westpac (2.5 per cent, including St George).NAB's growth was not as strong at the high-value end of the market. Roy Morgan said customers in the top quintile (based on the value of deposit accounts, loans, credit cards, wealth management and superannuation) accounted for 63 per cent of retail market value and generated the most profit for retail banks.NAB's customer growth in the top quintile over the four years was 9.2 per cent, followed by ANZ (8.8 per cent), CBA (3.9 per cent) and Westpac (2.8 per cent).NAB's strongest showing was in the bottom quintile (representing less than one per cent of market value), where its numbers grew 32.5 per cent, followed by CBA (5.8 per cent), Westpac (5.4 per cent). ANZ recorded a fall in numbers in this segment - down 6.9 per cent.