In Dragon Ball Heroes, Xeno Goku can turn Super Saiyan 4 and regular Goku can turn super saiyan blue. Could regular Goku or Goku Black turn into Super Saiyan 4 with proper training, or is there some condition that Xeno Goku has that makes him , Xeno Vegeta and Broly being able to turn Super Saiyan 4?
It is possible for other Goku to become super Saiyan 4. According to Fandom Wiki about Super Saiyan 4,
The only way a Saiyan can attain Super Saiyan 4 is if they have a high enough power level and become a Golden Great Ape (or Broly's variant) and regain conscious control over the form. Even after this form is attained, it cannot be accessed at will and requires exposure to Blutz Waves every time the user wishes to transform, however, Goku is apparently a special case, capable of using the form whenever he wishes. In Dragon Ball Heroes Gohan acquired the form without a tail while training and Xeno Trunks was originally going to have acquired the form too showing that even Half-Saiyans born without tails can still acquire Super Saiyan 4.
The form is a combination of the Super Saiyan and Great Ape forms attained through the Golden Great Ape form, it gifts the Saiyan with primal power, though they retain a humanoid appearance. It is possibly the Legendary Super Saiyan that appears once every 1,000 years.
Another possibility is that anyone can claim a wish after collecting dragon ball to get back Goku tail, well it is possible without a tail too, but with tail, it is much easier.