Don't forget the lich's cult members. 150 commoner-level cultists who beleive the lich is their arisen god, and who will gladly kill and die for their deity.

If the characters are on foot then they get mobbed. Each round, 6 cultists attempt to grapple them while 6 more stab them with pikes or long spears. While this is going on, the lich stands back, a long way back, raining ranged death on the party.

While a commoner isn't much threat to a tier 4 character, get enough of them and the natural 20s start to accumulate. Alternatively, give them all vials of fire and acid. 1d4 fire damage adds up when 100 cultists have just thrown Greek Fire at you.

There are more powerful cultists, of course, and some of them have magical power. Not a lot of power, but enough to hinder tier 4 characters. Dispel magic to take care of flying and warding. Silence and darkness, to hinder everyone.

Many of the cultists have ranged attacks, and they are arranged in groups all over the battlefield so that one fireball won't take them all out. Again, those nat 20s add up. To make it even nastier, make some of them level 1 wizards and give them magic missile scrolls and/or wands.

A lich has had plenty of time to grow wealth to buy this stuff. Several hundred vials of acid and fire plus tens of scrolls of magic missile is chump change.

:-)

A lich should be one of the toughest foes for a D&D party. First, they are old, really old. Second, they are smart, genius level smart. Any plan the characters come up with , the lich came up with it a century ago, spent 10 years thinking how to counter it, then spent another 10 years implementing the counter.

But... they are probably also insane. :-) They end up being defeated by their own paranoias, delusions, and psychoses.

"My cultists are holding off the adenturers. Another minute and the paladins will be dead. Or, I could fireball all my cultists. MOUAHAHAHAHAFIREBALL!!!"