We picked up a new game a few weeks ago called Hard Vacuum. Its based around space combat circa WWII. Physics is a little shaky but the kids and I love it.
(My wife isn't sold on it as she had a good start, but the kids have gotten REALLY good...)
Not 4x, strickly just scenario based, although the kids want me to come up with some way to make a campaign possible. You can design your own 'planes/ships' which the kids use to modify the existing designs to their liking. The booklets are small but the text is good. There were a few editing issues but the middle boy found a site with errata to fix it. The supplement 'HV: Science Gone Mad' has the errata in it (at least ours did). One of the kids has also downloaded another supplement (free). Straight forward and explanatory rules with plenty of detail if you want it, or you can leave it out if you want a quick and easy game. The supplement adds a bunch of weapons and options and is worth it if you like the game to begin with.
The basic game is just the US vs Germany in the space near earth. The supplement adds in the British. The download supplement adds in the Russians. It does not have a set scale, and you will need to supply your own hex map. The booklet comes with laminated counters that you would have to cut out yourself, but we just scanned them and adjusted the size to fit our map, then printed them on cardstock. You could use plain paper and be just fine. The rules allow for tabletop play without a hex field, and it works pretty well (we've done one big game on the kitchen floor) if you don't have spare hex mats laying around. All you really need is a ruler, although a protractor would help in a few cases. Makes it trickier as you can't just count the hexes.
Ships are quasi historical based with neat little blurbs about the ship's use and history in the battle for orbit, etc. Just reading the 'history' of the war and the ships if pretty good in and of itself (but I've always had a soft spot for that sort of stuff, and I'm definitely a big WWII fan). The designs aren't necessarily balanced, but the scenarios in the books are pretty well balanced. There is no incentive to design anything but a fighter in the rules, but there are designs for freighters. The supplement has bombers also, but other than victory points for a scenario there is no difference between a ship carrying one bomb or six. For the most part each nations ships is based around a different form of fuel or propulsion, with some variation in weapons based on the 'drive', but 13mm MGs, 20mm, 30mm, 75mm, rocket banks, mines, etc can always be put in and work rather well.
The biggest plus and funnest part of the whole game is the movement system. It is inertia based with each ship having a thruster 'diagram' allowing for different levels of thrust in the six different directions of the hex map, plus spin. Values carry over from turn to turn and are modified by the thrusters you fire in this turn, which can make guessing where your opponent is going to end up pretty tricky. On the tabletop just guessing where you will end up can be a little tricky. Trying to line up a shot on a bunch of wildly moving targets is actually pretty fun, as is pulling off some unusual manuevers the leave folks pointing the wrong way. My favorite is to try and overshoot my target while spinning the ship 180 degrees and firing after I've overshot my target. It is a blast. Of course when my target has guessed what I'm up to and has spun so that it is also flying 'backwards', my little tactic isn't quite so successful. This aspect of movement makes it completely unsuited for solataire play, as much of the fun is the 'dogfighting' aspect, which is lost when you control both sides of the battle. The various thrust values are tracked by little triangular chits that surround or are put on your counter, which actually works better than I thought. Even the 10 year old has figured it out pretty well, although he has occasional math errors (some of which I am sure are intentional).
Overall, I and my family have enjoyed this game. My kids tell me that there are two other supplements out there which add in the Japanese and the end of the war in another. If anyone knows where we could pick up a copy of these we would be very grateful. If you've got questions or comments, send them on.