see this table

There is a range to the answer for that question. based on level 4 agent quality it can be as low as 5.0 or as high as 7.0 standing. The table linked above shows you the agent level against the quality of the agent. simply check the field for the required standing. keep in mind that your standing can be slightly adjusted with the relevant social skills. if you need help finding a particular agent, say a low quality agent of a certain corp or faction to get into level 4s, try Chribba's Agent Finder

for a more complete breakdown try the Evelopedia mission guide