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."
We would normally say "Four types of gas". We are referring to more than one type, so we use the plural; but these are all members of the single class 'gas', so that is in the singular.
The plural is needed, because the number "four" shows that there is more than one. The plural of "type" is "types".
So, "four types of gases" is correct.
(Funnily, in the title of the question, the correct form was posted twice :-)