How to replace the first occurence in an expression instead of replacing all?

Well here's a way. Find the position of the first occurrence of x:

expr = HoldForm[x + 2 + 4 + x];

pos = Position[expr, x, -1, 1];

Then:

ReplacePart[expr, pos -> 4]

4 + 2 + 4 + x

Another way:

hf = HoldForm[x + 2 + 4 + x]
i = 0
hf /. (x :> 4 /; i++ == 0)

4 + 2 + 4 + x

Edit

For order preserving as Jens says, I changed Attributes

ClearAttributes[Plus, Orderless]

HoldForm[7 + x + 2 + 4 + x + 5] /. f___ + x + l___ :> f + 4 + l

7 + 4 + 2 + 4 + x + 5

And you can revert by SetAttributes[Plus, Orderless]

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Origin

How about this

HoldForm[x + 2 + 4 + x] /. x + a___ -> 4 + a

4 + 2 + 4 + x