We have a client with 4 domains, all of which are targeted to different countries. 3 domains are in the same language and share 80 - 90 % of the pages (80 - 90 % of the pages are the same on those 3 domains as they have information on it which is relevant to all countries). One of the domains is in a second language.
Now those 4 sites are all linked to each other (with nice little country flags). Would the pagerank on Google be better if the sites wouldn't have any reciprocal links? Or is it even a disadvantage to have those kind of links?
Would you recommend that the sites only link from A to B and C from B to C and D from C to D and from D to A?
I don't think the different TLDS have any influence on how Google deals with this. There are two points to look at: duplicate issues and crosslinking issues. Generally it's not recommendable to have the same content on different pages. I don't neccessarily suppose Google would catch on that and penalize or not rank one of the versions. It might happen though. Interlinking all sites probably increases the risk. To stay clear of this I would suggest to make the pages as different as possible.
Crosslinking also is not without risk. Lots has been written about that here on the board, I would suggest to do a site search and read some. No hard rules here, but lotsof educated opinions.