The update, which is available from UK distributor Positive Focus, is free of charge to Switch 07 users, while prices range from £585 to £3,900 excluding VAT for new customers. Annual maintenance is 20% of software cost, payable from installation.

Update 4 is a maintenance upgrade, issued in line with Gradual's policy of regular updating to fix inconsistencies in the software. Upgrades are issued about every eight weeks, providing a regular opportunity to introduce new and improved features and respond to changes in third-party products.

Positive Focus managing director Ivor Dixon said: "This is an important change that we, and Mac users, have been waiting for, so we can now take advantage of the latest generation of computers from Apple, with faster processors in both desktop and server products.

"Switch flows can run on both Mac and PC platforms, and all third-party applications need to run on the same physical computer as FullSwitch or PowerSwitch. While most major compatible applications, like those from Callas, have already been updated to run under Leopard, not all of those currently used have, so we suggest existing users look carefully at the flow they are using before considering changes to the operating system."

Among the fixes and optimisations included, the previous limit on the number of simultaneously active flows has been eliminated.