Printing photos one per sheet wastes most of the paper. A grid puts four, six or nine on a page.
An image that looks fine on a screen can be visibly soft on paper, because print packs many more dots into the same inch. Save your grid at the highest resolution you can rather than at a size that looked acceptable on the monitor.
Remember each photo shrinks. A photo printed four to a page is a quarter of the size it would be alone, so fine detail that was already marginal disappears. Choose pictures that read well small.
Leave margins. Most home printers cannot print to the edge of the paper, and a grid designed edge to edge comes out with one row clipped.
An A4 sheet at 300 dots per inch is about 2480 pixels wide. With two photos across and each photo set to 1200 pixels or more, the collage is wide enough to print sharply at that size. Set a gap of 20 or 30 pixels to leave a margin you can cut along.
Save as PNG or high quality JPG, and print at actual size or fit to page from your printer settings. On a standard sheet, two across and three down gives six photos at a usable size.
Four is comfortable for reasonable size, nine works if the photos are simple and bold.
Yes, leave a small gap between images to cut along.
No.
With two across, set each photo to 1200 to 1400 pixels. The finished collage is then wide enough to print sharply on A4.