Well, basically a Mark IV planet is pretty dangerous. Its called that because (sorry if this is too obvious) it contains Mark IV ships, which have four times the health AND four times the firepower of Mk I ships. So they can tear up your ships pretty quickly if you let them get outnumbered. To answer your second question, once you've sent non-scout ships to an AI world and the AI has "woken up" ships to attack them, many of those ships will become "threat" and become free to move around the galaxy and stir up trouble rather than hanging around the structures they were guarding originally --- the total number of these ships is the thing counted by the threat counter at the top-right of your screen. You may not have noticed them while attacking weaker worlds, partly because you may have managed to destroy almost all of the ships on them, but also because the AI doesn't like to waste threat ships and won't send them to attack you until it thinks it has a chance of overpowering your defenses --- which can happen pretty easily when you free up Mark IV ships as threat. Once there's enough threat ships to take on your worlds, you can expect them to start coming for you pretty fast.
Attacking high-mark worlds early game can be quite challenging, but there are a lot of strategies that can help you pull it off. If you're trying to do a gate-raid, the best thing is often to send a small strike force of raid starships or something similar, as smaller attacking forces will free less ships. If you want to take the whole world, you can also take it a little bit at a time, keeping your ships together so you can fight the detachment at each guardpost separate with better odds. You can also try to sneak up on them --- if you transport ships in before taking any of the bordering worlds, the planet won't be put on alert and it will have a lot less reenforcement. But for all these it's probably worth beefing up your defenses to make sure they can handle any Mk IV ships that do slip through --- and take the hit if you lose your fleet and the remaining ships go after you.
If you want to get some better guidance about this, there's an attack on a Mk IV world in the final tutorial scenario that tends to involve a pretty big threat counterattack, and there's a great deal of excellent advice about how to do it in a thread I started when I got stuck on that very part: <
http://www.arcengames.com/forums/index.php/topic,7514.0.html >
