Buy a 4 prong cord and plug set of the required size at the hardware.
(NEMA number of the outlet.)
Your old 3 prong dryer Had connections fir: Line A - (Black), Line B -(Red). Neutral, and Ground', and bonded the 'Neutral' and 'Ground' connections together.
There will be instructions with the cord about separating the two, but
as long as you get the 'Line' connections right, and the other two are solid
they rarely make a problem if not totally separated.
There is only a small additional safety margin in isolating Neutral from
ground as both conductors are linked at the Electrical service point
Retired Electrical Consultant