I do not know what it is.
DO YOU ???????
4mts tall
2 mt spread
stem striped 60 mm dia
very large leaves

someone here will know.
welcome to GOY

Right, whatever it is, it's definitely not Japanese Knotweed. Would like to see another pic once those flowers are open, please. Unless they're berries? Not too clear from the pic.

If they're not flowers, but more like whitish seed heads or berries, I suspect you might have a form of Poplar growing there, in particular Populus lasiocarpa. Google it, see what you think - if it is poplar, it might be suckered growth from a previous tree, or be coming from a large version nearby. Either way, if its anywhere near your house, and you decide it is the Poplar, get rid of it - they require huge amounts of water and are known to cause subsidence.