Hi everybody, 

here my noobie question : 

how can I have an horizontal symmetry between  two points on the left (the reference) and two others on the right ? 

Thanks for answering :)

Here's my suggestion.

Use the Node tool [n] to select all four nodes.
Open the Align and Distribute dialog [shift+ctrl+a].
    [Align Tab] [Distribute nodes horizontally]
Select the top two nodes.
Near the far right end of the Tool Controls Bar, turn on [Show transformation handles].
[shift+click+drag] one of the handle to change the node spacing symmetrically.

Thanks a lot @Paddy_CAD, that's what I exactly looking for :)

You may also use mirror symmetry lpe.