Welcome to the forum
@Claire it will be great having you on board
Like you my partner Val and I made the swop over from a large 6 x 2 x 2.5 , 220 gallon imperial indoor koi tank to the large outdoors after 22 years of keeping indoors.
At 4ft deep you are pushing it a tad as it is recommended that koi ponds should be between 4.5ft - 5ft in depth but you should be OK .
Forget everything you ever knew about aquarium keeping because as I found whilst waiting for the pond to mature it goes right against the grain as we know it especially waiting for the Ammonia and Nitrate to peak but after that it should be plain sailing for you.
If you go up to the articles section at the top of the page you will find a number of articles that we did to help budding koi keepers with everything from what books to buy, over wintering for the first time, alternate foods, what vitamins they need etc etc you get my drift
As you can see we are Brits
we are a total mixed bag of fish keepers here hailing from places like the UK , USA , Australia, New Zealand , India, Europe ,etc etc, the list goes on in fact we have members from around the globe all of us with varying levels of Knowledge about both goldfish and Koi.
Val and I are what are termed as Senior Aquarist's here in the UK with around a total of 30+ years of koi keeping.
We like the health side of the hobby the best as it opens up a world within a world of Parasites Bacteria and viruses so I suggest strongly that you purchase a microscope as it will be needed by you at a later date, this is to check your koi at the start of the season for parasites which all ponds have to varying degrees, now the trick is keeping them in check .
You'll need a good water test kit , many here use the API Pond Master test kit you just cant beat it, in fact I prefare it to the old Tetra Test kits that I used for many a year
Best of luck and the best of British to you as you start your new adventure .
Dave 54