not for me.
I'd rather take a named ringwraith on fellbeast for only a little bit more points.
And how do you get 10 attacks?
I'd rather think 8 on the charge.

I may be wrong but 3 base warg has 1 base = 4 plus 1 for charging = 5 then double from knockdown

You are mistaken only because the warg's one attack is not added into the equation. You use either the rider or the mounts attacks

As JamesR said, you don't add the one from the Warg, so you only get 4 attacks.

You also don't double the attacks to 8. What you get if you win and knockdown are 4 attacks, each of which gets 2 dice to wound. So you can only kill a max of 4 models, though in theory you could do 8 wounds if those models are heroes.

Im still confused about this. I swear Ive seen battle reports where a mount has more but a models not trapped. Can Gothmog ever have more than 4 attacks and in which case?

Also I dont get it.
You charge, win a fight, knock down, then you get rolls to wound in a turn right? 4 attacks to win the fight, then six you won, he falls and you double....?

What do you mean by "then six you won"? You get 4 attacks to win the fight. If you win, your opponent's models are knocked down. If you charged 6 WoMT, you could only kill four: you make 4 strikes, and for each strike you roll 2 dice. If, however, you charged Aragorn, you still make 4 strikes, but each strike gives you 2 dice, so you could still generate 8 wounds.

This is all, of course, assuming you are charging and in base contact with only infantry. If you are charging cavalry, or get counter-charged by cavalry before the fight is resolved, you don't get the extra attack, nor knockdown if you win.