I think this should work. Please correct me if I am wrong

UPDATE [MyTable]
SET [Field1] = @Field1,
        [Field2] = @Field2,
        [Field3] = CASE WHEN [Field4] THEN [Field3] ELSE @Field3 END

>I think the answer to this is really simple but right now I cant see the forest for all the trees
>
>consider the update statement
>

>UPDATE [MyTable]
>SET [Field1] = @Field1,
>        [Field2] = @Field2,
>        [Field3] = if Field4 is true then [Field3] else @Field3
>

>
>In other words if Field4 is true (it is a bit field) then I do not want to update Field3, but if Field4 is false then I want to update Field3 with the passed parameter for Field3.
>
>Any help would be awesome