Transaldolase that catalyzes the final step in 4-fluorothreonine biosynthesis. Mediates a L-threonine/fluoroaceldehyde to 4-fluoro-L-threonine/acetaldehyde crossover reaction. Can also convert chloroacetaldehyde into 4-chloro-L-threonine. Does not use glycine as a substrate. fluoroacetaldehyde + L-threonine = 4-fluoro-L-threonine + acetaldehyde Belongs to the SHMT family.