If anyone has come across this issue and found a way around it please post here as I would still like to know.
In the mean time I have downloaded the Yahoo mail app from Play and it has solved the issue. I can log in with my yahoo biz mail account and additionally switch between the personal & biz yahoo accounts. It's actually better than the standard mail app that comes with the SG4, for me anyway.
Hope this helps for anyone else who has come across this issue.