Which version works in this context best?

Hi,

(1) This application requires a Pentium 4, at a minimum
(2) This application requires a Pentium 4, at the minimum
(3) This application requires a Pentium 4, at the least
(4) This application requires a Pentium 4, as a minimum

I wonder if all of the above sentences are ok (grammarwise)?
Which version would you prefer in a similar context?

Could you tell me if (3) means exactly the same as (1) or (4)?

Can I put "at a minimum", "as a minimum", etc. immediately after the predicate (...requires)?

Can I put "at a minimum", "as a minimum", etc. immediately before the predicate (...requires)? (without changing its original meaning?)

Thank you so much!

mus-te