With SketchApp, I used A4 artboard preset and exported to PDF.
When i see print preview of that pdf, it shows like this:
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Normally printers can't print to the edge of the paper. There is always a white margin.
When choosing Scale to Fit, your document is scaled down to fit within those margins.
If you instead choose not to scale it (100%), your document will be printed at the size you intended, but since the printer can't print to the edge, your document will be cropped at the edges.
If you need to print the full A4 to the edge of the paper, you need to print on a larger piece of paper (A4 oversize or A3) and then afterwards cut it with a knife. This is how commercial print is normally done.
You need to add 3 mm bleed to your document. Since it is impossible to print with 100% accuracy, you cannot have objects at the edge of the page. So you have to make sure that these objects exceeds the paper by 3 mm. If you fail to do this, you might get thin white stripes without print at the edge of the paper after cutting.
You also need crop marks, to cut by. I'm not familiar with SketchApp but, crop marks can normally be added automatically when exporting a PDF.
When cutting, remember to only cut out the A4 rectangle. If you cut all the way through the paper, you'll loose the other crop marks.