DD is 3 and a bit...will do 4 year old pre school next year and start school in 2016.
They do play based learning...but, you need to remember things like painting...helps them learn how to hold a pencil and increases hand strength (as does playing on money bars and climbing and digging in a sandpit), taking turns and sharing are vital life skills, packing and unpacking their bags and opening food...all vital skills. Counting in books and knowing how books work (ie stories flow in order, pictures matching words, predicting endings) prepare them for reading and maths.
What seems like play...are the foundations of all other skills.
DD is already showing an interest in reading (like her big brother did lol) and we use alphabet story books and she has started playing on Reading Eggs (that also have a maths component) because she wants to. She knows her numbers and most of her letters and is able to recognise and write her name...but that is because she is interested so we do stuff together. She would tell you she is playing too...because it is always fun and she does it with one of us.
Pre school's have a limited time and should focus on the the important social structure and also how to behave in a classroom environment. If your kid is ready and asking for more, do some fun learning together...but remember to make it fun as it is not essential.
Even once they are at school, they spend way more time at home with you so if you want to support your kids make the time to help them learn.