Hi Joey !! You can follow Emir`s method or you can follow this one. Just in case you find it annoying to press tab and then 4 again and again if you want to open sculpt mode then follow this one ;)
STEP 1: Edit>preferences>keymap>check the option 'tab to open pie menu' (as Emir said)
STEP 2: The quickest way to set a shortcut key in Blender is to right-click the operator(in your case the operator is sculpt mode) you want to set a shortcut for it so select 'Assign Shortcut' now press the key combination you want as a shortcut for that operator(eg. C)
NOTE: the key that you will press will be your future shortcut for sculpt mode. You can't assign it with a shortcut like 'tab' 'ctrl' or 'shift'.I would recommend the shortcut to be one of the alphabets eg.'S'('S' for sculpt mode as it so easier to remember). Also if you move your mouse away from the 'assigned shortcut' option then it will disappear.
How do I check whether I have assigned my shortcut?
you can ensure by selecting the dropdown menu and beside sculpting mode, your desired shortcut key should be there in grey text.
REMEMBER STEP 1 MANDATORY
I hope this answer adds up to something useful and is comprehensible for you
How to reset a shortcut key in Blender?
You can reset a shortcut key in the user preferences. Go to Edit->Preferences and find the keymap section. If a shortcut has been changed from the defaults, a restore button will appear just above the shortcut key on the right side in the user interface. Press it to restore the operator to its default hotkey.
