1.
four hundred (400)
The number 400. 四 + 百, with no sound change.
四 + 百. Both parts keep their normal shape.
Sound changes in the hundreds
Only three of the hundreds change:
All the others, including 四百, are regular.
Pronunciation
The older reading しひゃく survives only in a few set expressions such as 四百四病 (the ills that flesh is heir to). For counting, always say よんひゃく.