Seamless texture on A4 canvas

[rich2005]

Patterns like your planks (brickwork is the same) is tileable on the vertical edges, the top and bottom finishing on a half-depth joint. One of the reason why tile seamless is not too good.

That pattern is 800 x 800 pix @ 120 ppi. There is no 'A4' size. The Gimp default A4 is 210x297 mm @ 300 ppi

You can have an 'A4' proportion, 800 x 1131 pix or if you are intending to print a size that fits the Gimp A4 is 827 x 1169 pix.

It might go like this, 
Scale to 827 x 827 pix
Resize the canvas to 827 x 1169
Duplicate the layer and move to fill in the gap (with a nice matching pattern.)

   

Then an A4 fill looks like this

   

All very straight forward until you come to print. Gimp will enforce margins and an inkjet printer usually needs a bottom margin 10 - 12 mm for a feed.  

Pattern attached.