I think the symbolic model will just be showing the math steps:

(-2) = x + 4

-4 -4

(-6) = x

For the pictoral, you could probably use a number line with the left side of zero representing negative numbers, the right side positive.

(-8)--(-7)--(-6)--(-5)--(-4)--(-3)--(-2)--(-1)--0--1--2--3--4--5

(-8)--(-7)--(-6)--(-5)--(-4)--(-3)-- (-2)--(-1)-- 0--1--2--3--4--5 this represents (-2)

(-8)--(-7)--(-6)--(-5)--(-4)--(-3)--(-2)--(-1)--0 --1--2--3--4 --5 this represents 4

When you subtract you put the second piece (what is being subtracted) onto the LEFT side of the first piece and see where you end up

(-8)--(-7)--(-6)--(-5)--(-4)--(-3)-- (-2)--(-1)-- 0--1--2--3--4--5

--(1) --(2)-- (3) --(4)

(-8)--(-7)-- (-6)--(-5)--(-4)--(-3)--(-2)--(-1)--
0--1--2--3--4--5

This demonstrates, pictorally, that when you take away 4 from (-2) you end up wtih (-6)

Hope this helps