Is there a way I can scan an A4 photo at 4800 dpi?

It is likely to be a marketing restriction to encourage serious (or gullible) users to buy more expensive models. The limitation is likely to be built into the device drivers rather than the scanner itself. This means that using other applications is unlikely to resolve the issue since application software must use the supplied device drivers.

However, you are unlikely to ever need 4800dpi resolution unless you are scanning specialist items. I normally scan everyday A4 documents at 300dpi on my Epson V600 scanner, or 600dpi if I want to use the scans for OCR. I normally scan photographic prints at 1200dpi. If I go higher than that, I can see no noticeable improvement in image quality. The same was true when I used a Canon LiDE 80 scanner.

I suggest that you experiment and do some test scans at various resolutions. I would be surprised if you find any benefit in going higher than 1200dpi.

Oh my. Why on earth would you need to scan something at 4800 ppi?

That would give you a file of 39840x56160 or a 2,237 Megapixels... a really pro normal digital shoot has rougly 80 to 100 Megapixels (not two thousand).

The restrictions are likely for people do not freeze its computer or fill their hard drive with just 50 scans.

A normal photo can be scanned at 200-300 dpi, and they will be fine.

I would only use 4800 ppi to scan slides or negatives, probably a collectable postage stamp or if I wanted to forge bills... All thoose cases the phisical size is smaller than A4.

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Edited based on the coment.

I would use 1200ppi. It is a recomended scan for line art.

There is a chance your images have some of this line art, but at the end will be converted to a halftone because you will be scanning to RGB.

2400 ppi is probably too high even for this project. You are not scanning the original art, but the imperfections of the print and paper. That is not part of the original design.

One word: stitching.

Determine the largest area you can scan at maximum resolution, then plot out the document you wish to scan in slightly overlapping areas that size; scan them all, and feed them into stitching software (commonly used to produce high resolution panoramas and the like). You'll need a huge amount of RAM in your computer (32 GB wouldn't be any too much) for the stitching job, and might well find you'll need to stitch a few tiles at a time, then stitch the larger ones, to avoid overloading something, but eventually, you'll get your couple of gigapixels.

I have no idea how you'll display it, though...

Is there anyway to bypass this, how could I scan the entire A4 photo at 4800 dpi?

You cannot bypass the limitation. As Zeiss Ikon states, you can scan several pieces and stitch. However, my personal attempts at doing so have been disappointing in terms of both wasted time and image quality (stitching artifacts).

... would it not be a software restriction rather hardware related?

Your scanner has a Contact Image Sensor (CIS). The sensels run the width of the scanning area and are positioned close to the surface they are intended to scan. To achieve 4800 DPI, a portion of the sensor likely has a higher density of sensels. There would be no way to scan at 4800 DPI outside of that area.

Other scanner technology use lenses and small, linear sensors. The sensors have fixed size and sensel density. To achieve higher DPI, the scan area has to be reduced because the sensor cannot be changed.

Many scanners allow higher resolution settings than the hardware is actually capable of providing. This is done via interpolation. If such solutions are sufficient for your needs, you can achieve the same results by resizing lower DPI scans in any image editor.

Most prints do not provide detail beyond ~300-600 DPI. Since you are scanning toy boxes, most of what you would be capturing is the halftone pattern.

I'm scanning old toy boxes, which flattened are around A4 size. I need the huge files and with as much detail as possible because I am going to restore the torn areas and cracks in the color etc in Photoshop, so the bigger the image the better the restoration can be.

Assuming these boxes were printed using a 4-color offset press, there were probably printed at 72-300 LPI (that's lines per inch). For toy boxes, you might guess somewhere in the low end of that range, typically 120 LPI. That's the final print resolution. No matter how high your DPI scan, you won't capture any more detail than that ~120 LPI, because it isn't there.

LPI and DPI are not the same thing, of course, and you can get more detail at, say 600 DPI about the smoothness of the individual half-tone dots. That might help with Photoshop repair. But about that, you are really just capturing information about the non-print areas between the dots at a microscopic level.

See discussion here.