That has led some analysts to speculate that at least one of the missiles involved in the test failed to fire.
Iranian officials have made no comment on the photo.
The missile test was part of a show of force by Iran, drawing sharp responses from both the US and Israel amid escalating tensions over Iran's alleged nuclear weapons programme.
Photo experts say the fourth missile appears to have been added digitally, pointing to similarities in the smoke trail and dust clouds from two of the other missiles.
The photo arm of the French news agency AFP, which was one of the first to release the original image, said on Thursday the picture was "apparently digitally altered to replace the grounded missile and launcher with a fourth successfully launched missile".
Mark Fitzpatrick of the London-based International Institute for Strategic Studies told AFP: "It very much does appear that Iran doctored the photo to cover up what apparently was a misfiring of one of the missiles."
"The whole purpose of this testing was to send a signal, so Iran both exaggerated the capabilities of the missile in their prose and apparently doctored the photos as well," he said.
Gerard Issert, a technician at Granon, one of the largest photo laboratories in Paris, also told the agency that the photo was a "doctored image".
"Although the missiles weren't all equidistant from the camera, they're the same size in the picture," Issert said.