Scaling up is the same grid method with bigger squares. If your reference is 20 cm wide with 10 columns, each cell is 2 cm. On a 4 m wall with the same 10 columns, each cell becomes 40 cm — a scale factor of twenty.
Measure the wall first, then decide the number of columns, not the other way round. Round cell sizes (25 cm, 50 cm) are far easier to chalk accurately than awkward ones like 37.4 cm.
Use a chalk line for the long runs and a spirit level or laser for the first vertical; every other line references those two. Chalk wipes off, pencil does not — resist the temptation to snap the grid in graphite.
Work top-left to bottom-right so you never lean on wet paint, and step back after every two or three cells. Murals are read from a distance, so an error that is invisible up close can be obvious from across the room.
Keep the printed reference gridded and labelled in your hand, and mark each cell off as you finish it. On a large wall it is surprisingly easy to lose your place by one row.
Finally, block in all your mid tones across the whole wall before any detail anywhere. A half-finished mural that is evenly covered reads better than one perfect corner beside bare plaster.