I would like to know if is possible to print a digital drawing, just the outline, on watercolor paper.
I would like to do it to test different watercolor result on the same drawing without redraw the same subject and mixing digital drawing with watercolor.
Maximum paper size is A4 format.
Not sure if ink jet or laser could get a better result more similar to ink and witch one is water-resistant
Laser printer toner is water resistant -- more so than most paper. But the toner doesn't necessarily stick very well to papers other than office paper and specially coated materials. Toner can also look rather shiny. So this could be a good option for your tests of different effects, but less so for the mixed digital/watercolour pieces.
Inkjets have the advantage of using ink which will look much more like the ink you'd expect especially on watercolour paper. Some are water resistant, some aren't. I've got an epson that's over 10 years old now; the ink for that doesn't run on normal paper (though it does a little on photo paper). There's a very slight solvent smell as the print comes out. Printers I've used in the past have varied.
You'll have to test on any printer you have in mind. I suggest that you test on office paper using only the inks you want to use for your project -- probably just black, which would be a good thing as there should be fewer components to run (be sure to print using "black ink only" or a similar option; some printers use black and the coloured inks to produce black. You'll have to further test the printing on your watercolour paper, trying different paper options in the printer software.
If you're looking to buy a printer to do this, at least try to get a demo print on office paper and see what happens when you wet it. The availability of T-shirt print transfers might be a minor guide as T shirts have to be washed, but the transfers might bond the ink to some extent.
Be very careful about using any type of paper that is not specifically designed for it in a laser printer. Laser printers do not use ink, they use toner, which is adhered to the surface of the paper with a lot of heat. Paper created to be used inside an inkjet printer usually has a coating of some sort, which can actually melt inside your laser printer and ruin it. Laser printers are pretty expensive to repair after something like this happens (ask me how I know that...).
Although watercolor paper will not have any sort of coating on it, the toner may not adhere well to the surface of the paper, and the paper may not hold up well to the heat of a laser printer.
Also, I did a quick Google search to confirm what I was telling you about the coating on inkjet paper, and ran across this article, which talks about why inkjet paper has a coating. It says "Most inkjet papers have a special coating on them to help the paper absorb the right amount of ink. If the ink is allowed to sink too far into the paper, it can 'feather' or spread out, making the image look fuzzy. High quality inkjet paper is coated to keep the ink at an optimal level for crisp, clean prints from an inkjet printer."
Watercolor paper is specifically supposed to be absorbent, but as stated in the quote, inkjet inks are prone to feathering on uncoated paper. I would think this feathering would be even more prominent on a paper that is designed to be absorbent. As you state in your question, you want to explore mixing digital art with watercolors--but I am not sure that watercolor paper would provide you with the crisp line quality that is often meant when someone talks about digital artwork.
However, I will say that I know a lot of people who run all kinds of things through their inkjet printers--waxed paper, cloth that has been treated with starch, etc.--and it seems to do no harm to the printer itself. So, I think you are probably safe experimenting with an inkjet printer and watercolor paper. But I would stay away from trying similar experiments with a laser printer.
