Introducing new fish?

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Hello, well i have had two medium size gold fish for about 4 years now, living on there own in a medium sized tank. they have grown a bit since i moved them to a bigger tank.
But today, i bought two more goldfish, and i have put them in with the 4 year old goldfish. i was just wondering if this would be okay? with the 4 fish in the tank, it is a bit of a squeeze, but i don't have the money for a new tank.

i was told that the older fish may eat the younger ones as they are seen as 'invaders'

let me know, thank you
its a 16 liter tank, the original 2 gold fish are fine, ive had them for 4 years, they have grown fully (about 15 cm in length) the two new fish are about 10cm in length.
Just standard everyday goldfish. and one fancy with a fan tail?