Only saw this thread now so sorry if it's too late
I play a Corvette standard in drop A.. the above advice is correct around filing the nut
If that feels drastic, especially with brass, then just use a taperwound low string
D'addario do a .135t gauge that sounds stable in a 4-string.. I use the NYXL and you can buy the string on its' own
Any taperwound string around 130 or higher will do and the rest of the strings will fit no problem without mods