1- He thinks the work is below is dignity.
2- He thinks the work is beneath is dignity.
3- He thinks the work is far below is dignity.
4- He thinks the work is far beneath is dignity.
I think the fourth one is incorrect because beneath gives the sense of touch, doesn’t it?So, nothing can be far beneath, I think.
The sentence ‘he thinks the work is (far) beneath his dignity’ would be the most commonly used. Although ‘beneath your dignity’ is a set expression, ‘(far) below your dignity’ can be said too.
Something can be far beneath: you can look out of a plane at the mountains or the clouds far beneath, for example.