Just so you know I'm not completely talking out of my arse, I'm background checked and registered with Ofsted to allow me to work with and teach children, even though I more often than not teach adults (I did all that Ofsted stuff mainly because my girlfriend is a childminder, so that I could assist her if necessary without causing my GF any problems work-wise). Nevertheless, I have very often worked with the kids my GF looks after professionally, where she has to monitor their development, keep records of it and submit reports about it, so I know a fair bit about what is and is not a good idea with kids from about age two, up to around age seven.
At four years old, children are just starting to understand responsibility, but they are not always aware of consequences, and so the level of responsibility they can be trusted with is fairly minimal. The classic example of this is family pets, children often want a pet, and invariably say they will care for it in an attempt to persuade parents to get a pet, but that commitment often wavers very quickly. It's worth noting that we are talking about a creature's welfare with that example, which is not a trivial matter, and so if we extrapolate that to cover people's safety and welfare with weaponry (even airsoft weaponry), it shows that responsibility may not be fully there with young kids, even if they manage to persuade you that it might be.
That said, not all children develop at the same rate. Of course when not worrying about if their kids are normal, most parents proudly imagine their kids are amazing whenever they do or achieve something new, so it can often be difficult to know how bright your kid is in comparison to others, but generally speaking a parent will know if their kid is ready to learn something, because it will very quickly become apparent if their attention wavers the moment you start talking about something in more detail. So, if your child is inquisitive and curious about your airsoft guns, then by all means explain them to the child, but keep the following in mind...
You will get arguments from sociologists about whether young boys are predisposed to want to play with guns, but the likelihood is that deep in there somewhere, are the genetics of the hunter/gatherer which will make boys more inclined to be interested in weapons. It may not please those in favour of equal opportunities among the sexes to acknowledge that, but it is generally rather pointless to do battle with nature. Like it or not, your kids will be exposed to guns in the media, in computer games, via their friends at school, etc, etc. So it's probably not a bad idea to at least try to install some sort of responsible attitude towards weaponry. However, at the age of four, whilst most little lads like nothing better than running around with other kids pointing a toy cap gun at them and shouting 'pow, you're dead', the concept of death has very little meaning to a four year old, and such games are more about power and imagination than about blowing someone's head off. Thus it is a bad idea to prohibit or snatch toy guns from little boys, imagining they will somehow be shielded from the world. Rather than angrily proclaiming to them that 'guns are dangerous', you should emphasise that it is 'fun to play' and 'pretend', as this is what young kids are doing in exactly the same way as they do when playing with toy trains and cars. Where toy guns are concerned, it is the perfect way to introduce the notion of playing safely, by pointing out your airsoft weapons are 'daddy's toys which he plays and pretends with'. You can emphasise that you too find it fun to imagine and play, but that you have to follow rules and play responsibly, just as they do, because even toys can hurt someone if you don't play nicely with them.
So there is absolutely nothing wrong with you showing your children your airsoft guns, it might even provide some good lessons and help stimulate their imaginations, but you should be at pains to emphasise safety - you don't have to go all 'drill sergeant' on them - just a simple mention every once in a while is probably all that they are likely to understand anyway at a young age. With that in mind, perhaps get a secure cabinet you can put your weapons away in. After all, you probably tell your kids to put their toys away in a toy box, so why not set them a good example and do the same? Kids want to emulate their parents and learn by example, so there's an opportunity to impart a propensity for being tidy!
As with everything child-related, it's better to praise than to admonish, if you can manage it. So if you are teaching them how to play with nerf guns or water pistols or whatever, then praise them when they play safely and teach them some basic safety rules. Again, keep this simple, such as asking them not to point their toy guns at those who are not playing. What is helpful, is to buy toy guns which really do look like toys (i.e. very obviously not real looking in terms of colour), as this helps to emphasise the difference between toys and real guns, and helps to define the borders of play versus real life. Save the RIFs til later though, they're a bit too young to be handling them apart from when you explain what they are before locking them away (and very bloody securely, kids are devious and clever).
