Even in it's infant form, 4.x branch is more secure than the 3.x branch, it's filesystem and classes, framework has security at it's core where 3.x has "tacked on" security patches all over it like a frankenstein monster, not to mention XSS and Sql dump protections in the core like php filter_input(),strip_tags(), htmlspecialchars(), etc. These all come from learning from the 3.x branch and it's problems and php 5.x and HTML5 and CSS3 coming into play as the now technologies to be using, if you don't embrace these, you will get left behind, these technologies change very fast and you can't afford not to or you get lost.
A lot of the needed "applications" are now integrated into 4.x and you will find redundant applications on the transfer regardless.
But testing on a beta board is suggested and recommended even by ipb.