No worries.
In that volume with good water and plenty of food you will be surprised how big they will grow. You know those tiny blackmoors and telescopic eyes you see for sale at Garden Centres etc, I have got them to a foot long (including Tail) within 3 or 4 years.
So even with just 2 fish you can end up with 2 foot in total fish length in a 4 foot tank!
Officially with fancy goldfish though, the tail isn't included in the total measurement unlike with koi.
These were the last 2 when I sold up, both 18 Months old when photos taken, body length about 4-5" and tail about 4-5" on top. Another 2 years and a total length of 12" would have been quite achievable.
https://i.postimg.cc/xjWYVYCv/MVIMG-20191221-170206.jpg
https://i.postimg.cc/qvGdK3Gw/IMG-20191221-170449-2.jpg