Great Britain will add one more face to their team on Feb. 7, the day before starting their three-day Europe/Africa Group 1 tie in Tallinn.
Jocelyn Rae is the fourth member of Keothavong's first squad, with the Lawn Tennis Association (LTA) leaving their options open for the final slot.
Keothavong took over from Judy Murray as Fed Cup captain in December, and backed high-flying Konta as a real threat in the line-up.
"In Johanna Konta we are lucky to have a player who is at the very top of the game right now, and she will make a formidable opponent for any player," said Keothavong.
"Along with Jo we have fantastic experience in Heather and Laura, with nearly 40 Fed Cup rubbers between them, and Joss Rae gives us a strong doubles option.
"There are some great personalities in this line up and a robust team spirit will be key to supporting us through this tricky format.
"We are not underestimating the challenge in front of us, there are strong nations in this group, and our ambition has to be to progress and take this team where it belongs, into the World Group."
The draw for the Europe/Africa Zone Group 1 round-robin takes place on January 16, with matches held from Feb. 8 to 10.
Pool winners will then contest promotion play-offs on Feb. 11, with victorious nations then progressing to World Group II play-offs in April.