When you say 4 O'Clocks, I am going to presume you mean Mirabilis jalapa, 4 O'Clock Flower or Marvel of Peru, which is an ornamental plant native to Peru as opposed to the many native 4 O'Clock flowers in our region.

With that in mind, your question is similar to one asked previously through eXtension so I will share that link:
 
https://ask.extension.org/expert/questions/60842
 
In short, while the seeds are considered poisonous and certain chemicals can be isolated from the roots to kill fish, there is no indication that the plants themselves "poison" the soil nor cause any concern for planting vegetables where you have previously grown 4 O'Clocks.
 
For more information on some native 4 O'Clocks, including some that are considered weeds, check the following sites:
 
http://extension.usu.edu/rangeplants/htm/four-o-clock
http://www.nwcb.wa.gov/detail.asp?weed=93
http://www.fs.fed.us/wildflowers/plant-of-the-week/mirabilis_macfarlanei.shtml