To elaborate a bit on @Claudia-Frank 's explanation, here’s some code that shows how to do a replace on only the text in one or more selections:

find = 'a'
replace = 'A'
num_selections = editor.getSelections()
for sel_nbr in range(num_selections):
    start_pos = editor.getSelectionNStart(sel_nbr)
    end_pos = editor.getSelectionNEnd(sel_nbr)
    editor.replace(find, replace, 0, start_pos, end_pos)

as the help states it replaces the selected text but from your responds
I understand that you want to, let’s say’ select 4 lines and within this 4 lines
you want to do a search and replace, correct?

If this is the case you need to use the getSelection functions,
most probably getSelectionStart() and getSelectionEnd() and use your
replace with the returned start and end position.

Cheers
Claudia

Open the console and type

>>> help(editor.replaceSel)
Help on method replaceSel:

replaceSel(...) method of Npp.Editor instance
    replaceSel( (Editor)arg1, (object)text) -> None :
        Replace the selected text with the argument text.

The first argument, editor, can always be ignored because you call it from
an editor instance, so you need just to provide the text which should be used to replace the content.

Cheers
Claudia