My husband is very money orientated and all he can think about is the cost of going from 4 to 5. Flights, hotel room, days out, new car etc etc. I can bear the thought of that for the next 20 years.
Presuming you're not in financial difficulty, cost is what you make it. Holidays, we drive or we go for cheaper options (e.g., cheaper accommodation). AirBnB opens up lots more options and there are lots of places that offer three bedrooms in their accommodation options.
Days out, we used to get a family ticket which covers two adults and two children then buy an additional two child tickets although we have Max Cards now for a lot of attractions as we have two children with additional needs. We normally buy an annual pass for a local attraction as their shared gift "from Santa" which can be used for days out all year. In previous years Santa has brought National Trust membership, English Heritage membership, annual passes for the zoo, several of the local museums and attractions, and this year he's bringing a cinema membership.
I can't help on the car as there are six of us so we have a seven seater anyway.
Clothes and shoes are our biggest child-related expense, you're talking around the £300 mark to kit all four out with shoes, wellies, PE trainers, and football boots. I keep back what I can for hand-me-downs, for example DS1 has grown out of a lovely Zara coat that he's only had for a couple of months and it's still pristine so it's been bagged and stored for DS2 for next winter. I make use of the sales and will buy things in the next size to put by for when they're needed.