When you set up a tax group, you can make a tax authority taxable.

(from http://help.sage300.com/en-us/2023/classic/index.htm#../Subsystems/TX/Content/TaxServices/AboutOrderOfTaxAuthorities.htm)

About the Order of Tax Authorities in a Group

The position of tax authorities in a tax group is important for taxes that are taxable. If a tax is taxable, the tax amount is added to the base for calculating invoice tax for all authorities lower on the group list.

Thanks,The tax base is the same for the first 4 taxes, the only tax base that differs is the last one(t5).

Is there a way to sum T1-T4 first? then calculate t5 on the subtotal only?

That's a reporting issue so you'll have to do some Crystal Reports work but that shouldn't be hard to implement.