Is there a reputable company to use?
You can find a lot of hosting companies on the
Minecraft Forum Hosting Companies section. I'm not sure I'd trust a lot of the positive reviews because I get the impression the majority of them are "encouraged", but a large number of negative comments is often a bad sign. I'm looking for a new host at the minute so I've spent aaaaaages digging through this and to be honest there's no definitive absolute best company. It's like any other kind of company: some people sign up and have a great experience, some people have an awful time, and the majority who have an alright time don't say much.
If it doesn't work out are they easy to get out of?
All of them offer a monthly payment period, and for some of them the total cost isn't higher if you pay per month than if you pay per six or twelve month periods. Unless you get a really good advance payment discount go for a monthly deal and then it is much easier to back out of.
By and large yes, once you've paid you'll get an email with server details. If you're running Vanilla MC you pretty much just need to wait a few hours for the server to go live and that's it.
What's a decent price to pay on a monthly basis?
Will just renting 4 user space server be enough (like no memory limitations etc)?
This really depends. Most of the hosts charge you per amount of RAM. Some charge per slot, and a rare few charge X per amount of RAM and add a surcharge for increasing slots. There's not much option but to flick through the different sites and see what's on offer really. 750Mb should be loads for 3/4 people, I've been running a 350Mb server with 2-3 people and a load of mods and it has more or less coped, so 750Mb for 4 players unmodded should be fine.
The only suggestion I have is don't go with Kerplunc/Tree Puncher. They're expensive, the control panel is pants and there is a bit too much downtime and disruption, hence why I've been looking for a new host...
http://www.gamingdeluxe.co.uk/game-servers/minecraft
Very reliable and EXCELLENT service.
One of the better GSP's IMO.
I took a gander at GD but I can't find any mention of the amount of RAM they allocate per slot. You wouldn't know this would you?