You have to ensure you have all the latest Up2Dates for V4, backup config, Install V5, restore config, perform all Up2Dates, backup config, Install V6, restore config.
I can't provide any reasons not to upgrade and in fact there may be many that would support it.
I don't run in a corporate environment... I only use ASL on my home ADSL line. V4.027 is meeting my needs very nicely but I was just curious as to your motivation to upgrade.
One issue that I am not sure has been resolved is the implementation of QoS. There have been numerous threads in the past about QoS in V5.x claiming that it did not function properly.