Hello!

Excuse me, I speak English poorly

How to transform a 4-node rectangular-shaped object into Bezier line 2-node?

Thanks!

Welcome to InkscapeForum!

Hhmmm....I'm not sure exactly what you're asking. If you have this
and you want this
you just need to do a little node editing. If it's not already a path, first select the rectangle and do Path menu > Object to Path. Next, switch to the Node tool. Click on the line you want to remove. That will highlight 2 nodes. Hold Ctrl key while you delete.

But if you have this and you want this
such as in the situation where you've traced a raster image, or perhaps performed Stroke to Path, that can't be done at all.

In case you're interested in the 2nd illustration, and you haven't the situations I described, here's how I made it. Basically I did the same thing as in the first illustration, i.e. node editing, then I changed the stroke width to match the rectangle width.

Hopefully one of those will cover your question, but if not, please try and explain more clearly. If you're having trouble translating what you want to say, you could try one of these free online translators: http://www.bing.com/translator/?ref=SAL ... &mkt=en-US or http://translate.google.com/translate_t?hl=en. I think Bing does a better job than google, but either one would work. (There are a few more out there too, if you care to search. But in my opinion, these are the best.)