The T3 designator is Heat treated then cold worked. The designation also applies to Products that are cold worked for the primary purpose of improving their strength i.e. 2024-T36 . It also applies to products in which flattening or straightening is recognized in applicable specifications such as flat sheet which is Designated 2024-T3.
The T4 designator is solution heat treated and naturally aged to a substantially stable condition. This designation applies when the product is NOT cold worked after heat treatment. The designation also applies when applicable specifications do not recognize the effects of cold work in flattening and straightening operations. The T4 designator applies when the product is heat treated by the customer.
Typical mechanical properties for 2024 : 2024-0 Ultimate 26,000 psi yield 11,000 psi . 2024 T3 ultimate 65,000 psi yield 45,000psi. 2024 T4 ultimate 64,000 psi yield 42,000psi.
2024-O HT to -T42 is not normally required by engineering unless significant forming is required that CANNOT be safely accomplished [potential for cracks] in the -T3 temper.
Moderate forming in the -O [annealed] condition or [more preferred] in the -W temper [unstable temper within ~15–45-minutes after solution HT/Quench] can be done to tighter bend radii or greater total strain, than in the -T3 temper.
IF forming is done as recommended, then mechanical allowables for -T3 and -T42 tempers are quite similar and for practical considerations are more-or-less equal.
So it would appear that if you can use 2024 T3 you may not have to heat treat.