One of my female budgies laid 3eggs in the nestbox 10days ago.yestday i found 1more egg on the cage floor?

It seems it would be from another female. A budgie will lay a clutch, this usually being 2-8 eggs, however they lay one egg every 48 hours until the final egg is laid. If this egg is from the original laying female, she may be laying infertile eggs with the time span so far off, and it's not very common for mom to lay outside of the nest after already laying that many eggs, unless she didn't make it to the nest quick enough.

You can look at their vent, (where the egg comes out) a female who has newly laid an egg will be enlarged, slightly swollen. Their lower belly won't be firm, but will be soft. Sometimes nesting will trigger other females to do the same, it could be another one of your females who laid it, in return, those are usually infertile, and are usually laid by a hormonal response, instead of an egg resulted from mating. In the future to discourage that behavior, if you know who the male is, you would want to house your breeding pair separate, (not now since she has begun laying the eggs) otherwise, you can have overlaying of egg issues with your other females.

You can handle eggs, as long as you don't allow the egg to roll around if holding it in your palm, and wash your hand, birds aren't born with the appropriate scent glands to detect human scent, however germs, and bacteria from your hands can be transmitted into the egg via the eggshell.

I don't think the egg is from the same bird. If it was from the same bird then it would have ment she pushed it out of the nest. The egg is infertile sinse it was on the ground and sinse there was only one egg. If u get infertile eggs often u can sell them for $0.20 an egg to repile owners. Repti owners like small bird eggs as a protien food. I sell my quail eggs to many repti owners.