Why arent Qasim, Abdullah, Ibrahim and Mohsin not considered among imams?

Qasim, Abdullah were born to Khadijah from loins of Prophet, Ibrahim to Maryam, prophets slave. Muhsin was third son of Fatima and Ali. My question is Hassan, Hussain are taken as imams, why arent Qasim or abdullah or muhsin not considered among imams? Google search isnt giving anything. If the 12 imams are given thrones in hereafter, why arent the above 4 given thrones?