Lightroom's Print module presents the currently set page size and obtains dimensions from the print driver, as to how close to each edge of this page size the ink can extend. These dimensions may be the same for all edges, or they may be different.
LR allows page margins that are further away from the paper edge than these dimensions, but not closer to the paper edge.
If you set in the print driver a "borderless" printing mode, then the dimensions that the print driver advises to LR, will be zero. This does not immediately change the LR page margins, but it does give you the possibility of reducing those to zero also. So in that case the LR print margins, and the page size, are the same and therefore your 6x4 aspect ratio photo can completely fill the page.
Even if your photo was not cropped to exactly the shape of the cell, you can still check the "zoom to fill" option and LR will make sure its whole area is covered, auto trimming any overflow on the fly.
Note: with most printers, the standard "borderless" mode automatically expands the image info when sent to the printer, by a few %, so that it makes sure that any inaccuracies of paper alignment or of manufactured paper size, won't leave any unprinted white slivers on one or another edge. This expansion means that parts of your composition seen right at the page edge inside LR, will get pushed "beyond" the page edge when printed. It's as if your photo was "zoomed in" slightly. You might make your crop slightly looser than required, in compensation for this (whereby these added edges will act as "bleed" for trimming, in effect).
Some printer drivers also offer a true-to-scale "borderless" option, whereby you can accept that risk of misalignment, and send the image to printer scaled exactly as laid out in LR. This also means that if you have chosen an output PPI to match the printer's native addressable resolution, this will (unlike with the standard borderless option) get respected in the output,
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