IMB Bank has launched an off-market share buyback - the fourth in a series of buybacks intended to cancel its ordinary shares.The mutual bank said it would buy up to A$18 million of its ordinary shares through a voluntary tender process.The buyback price will be determined at the end of the tender process. The price will be at a discount of up to 14 per cent of the volume weighted average price of IMB shares over the ten days on which they have traded prior to October 20 ($5.46), adjusted for the movement in the ASX 200 index from October 20 to December 7.At June 30 it had 31.9 million shares on issue.For some years IMB has been trying to unwind its hybrid structure. It is a mutual approved deposit-taking institution and also a public company limited by guarantee. Its shares are traded on an exempt market operated by IMB.A consultant's review commissioned by IMB in 2012 said the ordinary shares did not sit comfortably with the mutual structure and were an expensive form of capital.The consultant said the hybrid structure limited IMB's ability to participate in the industry consolidation that was going on at the time, and continues today.It recommended buying back the shares in a series of steps and the bank has been doing that ever since, with varying degrees of success.Its last buyback was in March last year, when it paid $16 million to buy 2.99 million shares at a deemed market value of $5.35 a share.The 2.99 million shares represented 8.6 per cent of IMB shares on issue. Since the it started its buyback program in 2012 it has bought back eight million shares, representing 20 per cent of the issued share capital at the commencement of the program.