My experience is that, in general, 4e battlefield is more dynamic, with more interruption and more movement/status effects, but IRC can handle it. I have never played over Maptool.
I'll assume the context is always PC vs monster. The less PC you have, the less resource and roles they have, the more tricky it becomes.
Group vs Group -> Excellent result. But need to keep the fight interesting once the PC gains upper hand - either change the monsters, change the terrain, or both.
Group vs Minion Sea ™ -> You need to throw a LOT of minions. Like, several dozens for a typical group, esp. if PC get lots of AOE and extra attacks. Very fun once in a while.
Group vs Minions + Solo or a few stronger mob -> Quite fun, Force players to think and plan. Sometimes you can mix them so that PC cannot (initially) distinguish the different mobs.
Group vs Solo -> For earlier monsters (read: MM monsters), this will get boring _quickly_. However, new monsters (esp. higher level monsters) from expansions esp. Dragon and Dungon are more varied, more fun.
Group vs Solo + a few stronger mob -> Similiar to group, creature and battlefield dynamic makes a huge different to how it play out.
Personally, I think battlefield is almost as important as the critters, because now PC have better access to tactical effects but at the same time more limited such resources. Have them fight over pits, in a maze of trapped corridors, in a dying forest where both sides can push down trees...
It's very easy to mix and match monster powers in 4e, But of course the better you know your players the better the result will be. You are expected to release multiple monsters, and the monsters themselves are more varied. Monster templates are easier to apply now, but still need hand adjustment. Certainly a big improvement, you get to feat or BAB to worry about.
When playing over web, make sure you are throwing encounter of higher level - +3 or +4 are usually fine. Same level encounter are walk over.
