As most of the above go for 1 larger than you think you need - a 6/8 berth would be fine. Make sure you see it up so that you can check that all the poles are there. I would recommend you go around some camping stores and look at the tents available BEFORE you buy anything on e-bay.
I would advise getting a larger one than you need. Especially if you are camping in this country - you will inevitably encounter rain! One that you can add a porch onto is a great idea because you can cook in them and store everything that you don't want in the tent. We have the Kalahari 10 and its brilliant. There is a kalahari 5 that with a porch makes a lovely sized tent and it is very well made. It has two bedrooms - so one room is slightly larger than the other and then it has a living area. With the porch added though it gives you so much more space. Remember with four of you there will be cooking gear, clothes, towels, food .... Etc, etc .....
Have you been camping before? Are you outdoorsie type people or like a little bit of comfort? I ask because we like a little comfort but very happy camping. We tend to take far more stuff than we need but always use the space we have!! A tent which has a living area where you can put a carpet down made a massive difference for us as it made it so much more homely at night time and for the kids to play in the tent when necessary.
Don't buy anything that is too old, try to find out how often and in what conditions they have used the tent. Also pets - we bought one tent last year and all the bedrooms were covered in dog hair!!! Not nice!!!
As above you just wont fit in a four man. I wouldnt buy a tent with out seeing it up unless you can trust the seller. I did once buy one at a car boot sale for £10 and it was fantastic so you never know it could be the reverse. Look at on line videos showing you how to put it up as this is helpful, oh and you must practice then its far easier and stress free whilst on holiday.
Its a top tip re cooking, it can be done but you have to be certain about ventlation and risk of fire, so best done outside if possible
One thing we learnt was don't get a 4 person tent, even if there are 4 of you- it won't be big enough. We got an 8 person tent in the end, it's just right for us and all our stuff and has space to eat/sit indoors when it rains (never cook indoors though!). Tunnel tents are much easier to put up than dome tents. That's about the extent of my tent knowledge, we've only been camping once so far
We are a family of four, dh is 6'4 and kids 8&5. I have been told that outwell are v good tents and have a found some good ones for a decent price £100-150 on eBay, but as we are complete novices what do we need to know? Any advice or experience of buying tents on eBay in particular would be appreciated.