We all want the best of everything.
That is the debate about loose or not-so-loose integration of best-of-breed applications vs all-in-one CMS applications.
The challenge is installing 4 applications and having some "glueware" to get them to work nicely together. You'll have 4 user bases and 4 sets of groups and permissions. You'll need to upgrade 4 applications and make sure the "glueware" still works. You'll have 4 different search engines. You'll need to customize the look & feel/navigation/user interface more than once.
Tiki has an excellent wiki and excellent group/permission system. Articles, blogs & forums are very good. I can't think of some software which is significantly better than Tiki for these features. If you want to improve with "Nicer graphics/frontportal", you just need to make a nicer theme. Please see http://themes.tikiwiki.org
If you need something that Tiki is not very good at (ex.: webmail), then it makes sense to have a separate application. Please read this page:
Also, please remember that Tiki is improving all the time. So you can identify something which should be better, make it better or find someone to make it better.
In short, IMHO, it's less work to make a nice theme and to improve a few things in Tiki than to manage 4 applications.