Sed command to append below the specific text

I've a scenario where I want to add some content below specific line as shown below.

func (v version) string(){
	vers := [...]string{
		"4",
		"5",
       //"6",         //this should be added
		"vE",
	}

Above is my code block and I want to add "6", below "5" using sed operation.
I've seen multiple articles and post but couldn't get the correct answer.

What I've tried is

    current_number = '"5",'
    next_number = '"6",'    
    os.system("sed -i /"+current_number+"/a"+next_number+" sample.txt")

Edit 1 : After @steffen's answer I get below error, image attached.

It works:

$ sed '/"5",/a"6",' file
func (v version) string(){
    vers := [...]string{
        "4",
        "5",
"6",
        "vE",
    }

But I think you need to add single quotes. I think your command reads:

sed -i /"5",/a"6", sample.txt

And that's equivalent to:

sed -i /5,/a6, sample.txt

Here's how to use variables:

PATTERN='"5",'
NEWLINE='"6",'
sed "/$PATTERN/a$NEWLINE" file
sed '/'"$PATTERN"'/a'"$NEWLINE" file # alternatively

The latter uses an idiomatic format, ' for all the literals and " for variables.

Given these variables:

$ current_number='"5"'
$ next_number='"6"'

If you really want to use sed you could do the following to retain the indentation for the new line:

$ sed -n 'p; s/'"$current_number"'/'"$next_number"'/p' file
func (v version) string(){
    vers := [...]string{
        "4",
        "5",
        "6",
        "vE",
    }

but personally I'd use awk for ease of future updates, maintenance, etc:

$ awk -v old="$current_number" -v new="$next_number" '{print} sub(old,new)' file
func (v version) string(){
    vers := [...]string{
        "4",
        "5",
        "6",
        "vE",
    }

Use awk -i inplace ... with GNU awk like you're using sed -i ... with GNU sed to update the input file.