[Sidd]

~= is "not equal" in lua

you seem to be under the impression that ~= is equals.
In your code: A soldier is not a demoman, therefore multiply by 4
and a soldier is not a pyro, so multiply by 4 again

Fixed:

function player_ondamage( player, damageinfo )
	if player:GetClass() == Player.kSoldier then
	    local damageforce = damageinfo:GetDamageForce()
	    damageinfo:SetDamageForce(Vector( damageforce.x * 4, damageforce.y * 4, damageforce.z * 4))
	    damageinfo:SetDamage( 0 )
	elseif player:GetClass() == Player.kDemoman then
	    local damageforce = damageinfo:GetDamageForce()
	    damageinfo:SetDamageForce(Vector( damageforce.x * 4, damageforce.y * 4, damageforce.z * 4))
	    damageinfo:SetDamage( 0 )
	elseif player:GetClass() == Player.kPyro then
	    local damageforce = damageinfo:GetDamageForce()
	    damageinfo:SetDamageForce(Vector( damageforce.x * 4, damageforce.y * 4, damageforce.z * 4))
	    damageinfo:SetDamage( 0 )
	end
end

edit: silly lua should use elif. It's like sooo much shorter