Shall we use type or types after a number. For example: 'four types of gases' or 'four type of gases'. Which of these is correct?
The word type must be plural (with an 's') as you are referring to a total of four, which is not a singular amount.
A shorter alternative to this would be to state, 'four gas types':
"There are four gas types that must be included in your summary."
The word type could be used when you are referring to each gas as a separate, singular item:
"Each different type of gas must be accounted for in your summary."