We know when $D>4$, i.e. $D$ larger than [upper critical dimension](https://en.wikipedia.org/wiki/Critical_dimension), then critical exponents are exactly same as the ones of mean field . When $D<4$, critical exponents are not given correctly by the Landau theory. Many books will list criticial exponents like Ising model in $D=2$ and $D=3$. But it seems that textbooks don't talk about or list the critical exponents in $D=4$. 

Besides what's about RG flow in $D=4$. In class I've learnt the perturbative RG flow of Guassian model in $D<4$ and $D>4$ and we see their RG have totally different properties. But what's about $D=4$?

My questions:

1. For Ising model, Heisenberg model and $O(n)$ model, what are their critical exponents in $D=4$ (There should be numerical result). You could directly give me the reference.
2. What's special when $D=4$? There should be some reasons why normal textbooks avoid discussion of $D=4$.