1 added to largest 4-digit number gives:

Understanding place value and number systems is fundamental in mathematics. The largest 4-digit number is the highest possible number that can be represented using four digits.

• The largest 4-digit number is 9999. When 1 is added to 9999, the sum is 10000. This is the smallest 5-digit number, demonstrating the transition between number magnitudes.
• 9999 is incorrect as it is the largest 4-digit number itself, not the result of adding 1 to it.
• 9998 is incorrect as it is one less than the largest 4-digit number.
• 10001 is incorrect as it is two more than the largest 4-digit number.