This is very common question and have been asked many a times. I always keep on reading new stuff about the marketing strategies and reviews keeps on changing. There used to be the times when people buy different domains for localization purpose but now the scenario has been changed.
According to the query, you want your site in multi-language and therefore want changes in URLs for different places. When I suggest to go for multi-lingual website, what I mean basically is one website with different languages for the people accessing the websites from different location.
The situation here is that you are four different domains targeting one company with duplicate content but yes, you are getting good conversion rate, which is actually a good news. But, Google Algorithms will not take much time to convert the good news into bad one.
What I can suggest is to let it be like this until you see major changes in Google search results. But, yes I would have made a back up ready if at your place. I will definitely make 301 redirects from the existing domain in the web results to my new domain (I am talking about the scenario when Google Algorithms will affect your 4 domains).
301 redirection helps in getting about 90-99% of the link juice of the old domain. It will help in ranking the site again for he same keywords. You do not have to worry about the number of keywords getting ranks for all domains. Actually, it s good part that you are getting one keyword for all tactic and it is going good for the business. Just keep your back up ready for the bad time (if they come).
Now, let me show you other face of the coin where your customers will get annoyed with multiple domain taking them to one company only. In this way, you need to start working on Analytics to filter the traffic and remove the domain getting less traffic. In this way, you will have more conversion rates from 3 domain rather than 4. Keep on doing the same with others so that you leave with the final domain to work on. Well, it is just a suggestion but take steps only after going through all the parameters.