My experience was that 3e still required groups of monsters, even though the encounter building rules often said otherwise. Most monsters didn't really have anything to tip the economy of actions in their favor, so leaving sending them in ones or twos just meant they got mowed down since the PCs had so many more attacks. Most monsters didn't really seem able to stand alone without getting smeared.
And the ones that were able to fight effectively when heavily outnumbered tended to be really annoying for some people. Stuff like Dragons who were super tough spellcasting fliers. Monsters with off buttons for a few PCs. Flight so melee people have to waste actions getting to fly or use weak attacks. Grapple related stuff. Etc. You don't want to fight that stuff all the time.
This was my experience with 3e as well. When I DMed 3e my prep time was all about building a party of monsters from scratch since the monster books only provided a baseline. And adventure stat blocks were notoriously incorrect requiring a re-engineering to verify the math. Also a large chunk of prep time went to reviewing all the spell like abilities of the monsters.
In 4e my prep time went down because WotC has done all this work for me. The game has a variety of roles for each monster type, so I no longer have to build my own monsters. I also no longer spend time reverse engineering stat blocks to verify math, nor do I need to review spell like abilities. I thank the 4e design team every day for moving to self-contained monster stat blocks.
All that being said, here are some of the keys to winging a spur of the moment 4e fight that you hadn't prepared in advance.
1. Terrain - Don't break your back over trying to come up with unique terrain features every time. Just think about where the fight is taking place and throw something in there. If a battle breaks out in a throne room, you can scatter a few pillars around which make convenient obstacles affecting movement and cover. If the battle is in a forest, then throw in some trees to act as obstacles, and several squares of difficult terrain to represent roots and shrubs, maybe even a pool of water. That's all you need really. Shouldn't need to spend more than a couple of minutes coming up with something and sketching it out on the battlemat.
2. Encounter Mix - This really depends on the circumstances of your spur of the moment encounter but some basic ideas are as follows:
Stand up fight - 3 soldiers, 2 brutes or 2 skirmishers
Enemy commander present - 1 controller, 2 soldiers, and 2 brutes or 2 skirmishers
Ambush! - 2 skirmishers or lurkers, 3 artillery, 1 controller
3. Monsters - Again if you have something in mind, then great, flip open the monster manual run it out of the book, or reskin another monster. Otherwise you can take about 10 minutes to create your own monsters on the fly. Use the 4e DMG page 184 and 185 and a piece of scratch paper. Send your players to grab sodas or hit the restroom while you decide what you want to do.
The charts for making monsters on page 184-185 of the DMG are perfect for giving you baseline attack bonus, AC, and Defenses based on role and level. For damage, I game with a bunch of power gamers so I provide my on-the-fly monster with a basic attack that uses the Medium Normal Damage Expression and then I add +5 damage per tier. But if that's too much for you just pick Low, Medium, or High from the table without the added bonus damage.
I also may provide the monster with a more powerful ability that uses the Limited Damage Expression table and has a refresh 4,5, and/or 6 depending on whether I picked damage from the Low, Medium, or High column, respectively. As far as what the special attack looks like, well I just wing it!

I try to come up with a cool narrative description based on the particular monsters and the fight.
Then depending on the role, I provide an additional special ability which I have just simplified to be easier to remember and apply on the fly:
Skirmishers - Can shift when they move, do +5 more damage when attacking after moving (describe the movement in some cool narrative way based on the monster)
Brute - Always does +5 damage, maybe pushes as well.
Soldier - Can mark foes, adjacent allies have Combat Advantage against marked foes, maybe inflicts forced movement as well.
Lurker - Does +5 damage when has Combat Advantage, and gains Concealment until the beginning of its next turn during or after a move (described narratively by me based on the monster and the circumstances of the fight). Maybe phasing or invisibility depending on the monster.
Controller - Attacks are Range 20, or Burst 1 within 10 - Can also Daze, Slide 2 squares per tier, or inflict some other condition on a hit as appropriate. OR can heal allies 10 HP per tier in burst 5, or provide 10 Temp HP per tier in Burst 10 with a standard action or as a minor with refresh 5,6.
Artillery - Attacks are Range 20, or Burst 1 within 10.
Leader - If one monster is a leader in addition to its basic role, add +1 to all its attacks and defenses, and it does +5 more damage, OR grants combat advantage to adjacent allies, OR it can grant a basic attack to any one ally it can see as a standard or minor action depending on how tough you want it to be.
Also feel free to add the Elite, or Solo templates as needed. Or add an aura or resistance or vulnerability based on the monster. For example, undead should be vulnerable to radiant, and resistant to necrotic, and immune to poison. Fire creatures might exude a fiery aura 1 per tier that does 5 Fire per tier, and so on.
Thats it. I don't usually worry so much about minor actions, or adding a lot of different special abilities, but depending on the monster I might add something unique. For example, fighting some nameless tentacled horror in the depths of the large fantasy metropolis' dark sewer, I would allow it to grab on a successful hit, and then allow it to deal bonus damage to grabbed opponents on subsequent rounds, or something along those lines.
Anyway, one encounter and foes ready to go. Should only take you 10 to 15 minutes to whip that whole thing up. My players are 4e veterans and power gamers so I tend to err on the side of having my monsters nastier than normal, so of course adjust your on the fly monsters to match the toughness of your PCs.