They should be ok until the achara gets bigger. Then it will be a crap shoot. 4 line should max out around 8 - 10", the sailfin will get fatter and longer. Once there is 3 or 4 inch difference between them it could get risky.
Ive got a 14" achara with and 11" 4line with no problems. They also live with a 5" sun cat with no issues, I've found that my achara doesn't seem to be as predatory as other catfish, just prefers to wait for it's pellets.