Ireland is very accommodating of 4 year olds. As the mother of 3 boys, the youngest now 7, I've had (fairly) recent experience of travelling with a child of this age.
Almost every small village has a playground, so there's almost always somewhere to go for a swing/ round-about/ see-saw, and most importantly to meet other kids.
Ireland has a lot of castles and ruins, it's the perfect back-drop for most kids of that age to explore, climb and imagine. Buy them a wooden sword and shield, or a viking/ knight's helmet, widely available in many tourist shops, and they're kitted out. If you can't find a wooden or plastic sword, use a stick.
As an island we have fabulous beaches. If you're travelling earlier in the year (before June) it might be an idea to bring a wet-suit. Give them a bucket-and-spade, and they'll keep themselves busy.
Irish food served up in restaurants and cafes is often locally sourced, so it's fresh and tasty. Food can often be quite plain, in some places everything comes with chips, and you may want to avoid that. But portions are large and filling. If your child likes a particular kind of hummus or ketchup, perhaps bring that with you. The only potential downside, is that in some rural communities, food is only served in the evening in pubs, and children must be off the premises by 9pm.
Otherwise, I'd say bring your 4 year old child, and s/he'll have a brilliant time.