Highest and Lowest

As is often the case with solving problems, there are several ways of looking at the problem. For example, Mark from St Johns, Walsall Wood kept the digits separate and did:

"Highest: $3 \times4 \times5 \times6 = 360$

Lowest: $3 - 4 \times5 \times6 = - 117$"

But Jason from Dunstable put some digits together:

"I made the highest number $3258$ by simply multiplying $543$ by $6$.

I made the lowest number $-651$ by simply subtracting $654$ from $3$".