Iron Man to appear in Captain America 3

Robert Downey Jr. has had a bit of a rollercoaster lately. Rumors are circulating around whether or not there will be an Iron Man 4 and in light of that, it was announced tonight that he is on the verge of signing a contract that will place him in Captain America 3 in 2016.

If this happens it is a pretty good sign that Marvel is introducing the Civil War storyline which pits Captain America against Tony Stark as they feud over the Superhero Registration Act. The act forces all people and creatures with superhuman abilities to register and disclose their gifts with the government and agree to act as a police force for them. Stark, along with Mr. Fantastic and Hank Pym, agrees with the SRA while Capt. Rogers and his team does not. Cap cites it as a violation of civil liberties and believes superhumans need privacy in order to help whenever the law cannot, they are not meant to actually be the law.

Basically, if this is the way they go it would put Stark in a somewhat villainous role that could lead into him being a villain in the third and/or fourth Avengers movie as well as a fourth Iron Man, if that happens.
Marvel is already planning on rebooting the 2006 Civil War storyline in 2015 to re-introduce and bring in new audience members before Captain America 3. The original comic series included crossover characters such as Spider-Man, Mr. Fantastic, Luke Cage, Wolverine, and Cyclops.

This is somewhat intriguing as another rumor going around in the past week or so was that Sony and Marvel were doing some re-negotiating and that it was possible that Spider-Man could possibly make an appearance in the Marvel Cinematic Universe. There is also the case of a new X-Men film coming out in 2016 as well as the new Fantastic Four. If there is any possibility of a Marvel film crossover it very well could stem off Captain America 3 and extend into the third or fourth Avengers.

Iron Man 4 not planned as of right now

Since Iron Man 3 disappointed a majority audiences, many have been looking forward to a fourth Iron Man film to redeem the character. However, according to Robert Downey Jr., a fourth Iron Man is not currently planned and it's not on Marvel's priority list.

The theory as of right now is that Guardians of the Galaxy has changed a lot of Marvel's priorities. James Gunn's film was a much bigger hit than anyone expected and it's currently on track to hit $300 million worldwide and has already won the title of the biggest hit of 2014. The first round of Marvel characters we were introduced to is drawing to a close as newer, more exciting characters are moving in.
Previously, it seemed that Marvel was riding the wave of Iron Man and the popularity of Robert Downey Jr. but now that it's been proven that other characters and actors can bring just as much, if not more, to the table. Not to mention the newer additions don't carry the price tag that Downey does so they make more profit at the end of the day.

This doesn't necessarily mean there will never be an Iron Man 4, but it isn't looking very good at the moment. There are currently 11 open slots in the Marvel schedule until 2019 so there is still a chance Stark will venture out on his own.
I wasn't the biggest fan of Iron Man 3, but in the end it did lend a pretty good wrap for Tony Stark. However, we need to bear in mind that he will return in The Avengers: Age of Ultron in May, so the wrap-up they gave him will most likely be ruined.