Dismiss/Control +4 replaces Wedge Mastery, so you no longer have it on your card (and it's previously been stated that even if you have an OS on your card that is a replaced Pre-Req of another OS on your card for some reason - usually Database Glitch - you don't gain the benefits of it).
- In short, No they do not stack on a single player.

Both OS's also state "you may add 4 to the power of any dismiss or control wedge/they cast , in addition to any spell cards they use" - So they are free.

If one is from the character, and one from an item do they then stack?

Items will almost always specify that they grant the relevant OS, and thus do not stack.

In the highly unlikely event that they simply add +X then, as Jason says, they do stack.

Edit: Minor clarification added

Where the wording in the OS guide is "...allow the character to add up to n to the power..." (my emphasis) then it allows you to choose any integer amount between 0 and n.
Where it just says "...may add n to the power..>" then it always adds either exactly n or 0 (you may choose not to use the OS at all)

In summary:
Wedge Mastery adds exactly 4 when you are the focus of a wedge, or does nothing if you choose not to use it
Dismiss / Control +4 adds your choice of 0, 1, 2, 3, or 4 regardless of whether you are the focus of a wedge or not
Wedge Mastery (Improved) adds your choice of either exactly 8 or nothing when you are the focus of a wedge, or your choice of 0, 1, 2, 3, or 4 when you are not the focus of a wedge
Dismiss / Control +8 adds your choice of 0, 1, 2, 3, 4, 5, 6, 7, or 8 regardless of whether you are the focus of a wedge or not

Thanks Rich, I should have been clearer in my question. I was indeed talking about wedge mastery and improved wedge mastery at the head of a wedge.

I will copy your post to the incantors board.

Just for absolute clarity, do you need to declare a type of dismiss, in the same way as a type of control. eg dismiss rank x or dismiss deamon rank x?

cheers