I have a 4 bedroom 2bath 2400sq.ft. manufactured home with 5ton rated ac unit.Do I need this much or can I downsize.

You need to do a manual J to determine the correct amount of cooling required. All else is guess work.

I will state the obvious, the size of your house (2400 sq. ft.) would probably require a five ton system and very few contractors would put in a system too large for a house because it would cost them more money. If you want to save money on the operation of the A/C, then check into upgrading to a higher efficiency system. Call at least three A/C contractors in your area and they will be glad to give you a free quote.

((I will state the obvious))

For many years rule of thumb is 1 ton (12,000 BTUs) for every 600 square foot. That means he would actually need 4 ton with the rule of thumb figure.
The only real way to calculate is by manual J and with that you will get a accurate size. To small not enough cooling. To big and it will not run long enough to remove the moisture in the air causing it to be cold and clammy.

Always best to calculate and not guess!!

Contractor charge for the size of the unit they sell the bigger the unit the more profit weather it works correctly or not.