What is happening here?
In Grade 4 the number range grows to one million. The numbers do not get harder, only longer: 305,006 has six digits, and every digit has a name and a value.
The comma helps with reading. It splits the number into blocks of three from the right: ones block, thousands block, millions block. Each block is read like a small number with its name attached: "three hundred five thousand and six".
What helps at home
Look for big numbers in everyday life and read them out loud: populations, distances in meters, prices of cars, crowd sizes in a stadium. Then write them into the place-value chart and ask: what is in the hundred-thousands place?
A picture helps with the sense of size: 1,000 thousands make a million. A sheet of paper is about 0.1 mm thick, a stack of 1,000 sheets is about 10 cm high. How tall would a stack of a million sheets be? Questions like these make the size tangible.
What can go wrong
A stumbling block is zeros in the middle of a number. "Three hundred five thousand six" quickly turns into 3,056 or 305.60. Writing each block on its own (305 | 006) keeps every zero in place.
Many also slip when comparing: a number with more digits is always bigger, and with the same number of digits the first digit from the left that differs decides. When rounding, only the one digit to the right of the rounding place counts, and all digits after it turn into zeros.