1) 400kg takin vs 70kg snow leopard (stealth)
2) 400kg takin vs 350kg kelenken
3) 400kg takin vs 4x 20kg dhole
1) 400kg takin vs 70kg snow leopard (stealth): A takin is a stout-bodied bovid with short legs and a thick neck. Its horns curve out and back. Snow leopards are excellent hunters that are capable of bringing down animals over 3 times their own weight. They are quick, agile, athletic, and skilled at killing. A snow leopard is probably capable of asphyxiating a subadult takin with a throat bite, but won't have the size or strength to consistently succeed with a 400kg adult. This would be similar to a lioness trying to tackle a caffer buffalo by herself. Even with an ambush, the snow leopard's chances aren't good. 400kg takin wins.
2) 400kg takin vs 350kg Kelenken: The Kelenken was one of the "terror birds", a prehistoric predator as tall as an ostrich and significantly heavier. Kelenken had a huge, deep beak that was used to bite and possibly strike with, and it may have kicked adversaries much like modern ostriches and cassowaries do. Kelenken was probably a swift runner, too. The takin will only be half of the Kelenken's height, but will have a small weight advantage. Charging into Kelenken might topple the bird, but the takin will need to avoid the kicks from below and the bites & strikes from above. A lot of this depends on how practiced Kelenken was at taking on large animals (it may have targeted animals a lot smaller than takin) and how diversified its offense actually was (what if it didn't kick? what if it didn't strike with its beak?). If Kelenken was indeed adept at predating upon large animals and was capable of biting, striking, and kicking during an attack, it would probably defeat the takin more times than not. If Kelenken was the hunter of smaller game or lacked the entire offensive repertoire commonly assigned to it, it would probably lose to the heavier takin. Without knowing more, I'll call this a 50/50 at these weights.
3) 400kg takin vs 4x 20kg dhole: Dholes are nimble canids with strong bites. They are excellent pack hunters, and can bring down relatively large animals when they work together. Takins and dholes share habitats, so encounters are possible. The 4 dholes will try to attack from all sides to wear the takin down, and the takin will use its horns and hooves to defend itself. In this scenario the takin weighs 20 times as much as each dhole, which makes this similar to 4 grey wolves trying to bring down a water buffalo or a large bison. The dholes will need more members to be favored against this 400kg bovid. Takin wins.