I have found three sources, which all give fairly similar numbers. I'll present them here:
Committee to Protect Journalists
The Committee to Protect Journalists has an interactive map showing Journalist deaths grouped by country. The default criteria shown result in the USA being in the top 5, specifically tied with Mexico for 4th with 4 deaths. That filtering is "motive confirmed", which I don't know the precise criteria used. When expanding to include "motive unconfirmed" deaths as well, USA falls to 5th tied with India at 5 deaths.
International Federation of Journalists
The International Federation of Journalists consolidates a list of Journalist deaths annually. This year's list places the USA tied in 6th with 5 deaths, alongside Pakistan and Somalia.
Reporters Sans Frontières
Reporters Sans Frontières, or Reporters Without Borders, publishes a similar annual report, which also includes some other information about journalists such as ones which were detained rather than killed. This is the report cited by CNN, and indeed places the USA at 5th with 6 deaths, tied with India.
All three of these sources have slightly different numbers, so seem likely to have reached their conclusions through independent research and possibly slightly different criteria used for inclusion. Despite that variance, all three place USA between 4th and 6th place. Thus I would say it is very likely that CNN is using accurate data and slight disagreements about definitions are unlikely to make a large difference.
The source for this question additionally claims that this is the "first time" for the USA. I haven't checked this since the question doesn't mention it, but each of the sources I used seems to collect and publish this information each year so it should be possible to check; I haven't had the time yet.