L. B.
I have a 6 year old and twins that will be four soon... The picking up toys has been a battle for a long time. I will share with you what I have done and what seems to be working for me now... Every child is different and yours may respond earlier than mine did.
The first thing we tried was a "Monster Box". Any toys they don't clean up got eaten by the "Monster Box". This box was then put away until they "earned" the privilege of getting these toys back by keeping what was left cleaned up.
The monster box worked at first, and then my kids ended up with ALL their toys in the Monster Box... more than once. I got tired of being the one to put all the toys in the box.
Plan B. Love and Logic parenting classes teach you to simply say, "Please feel free to pick up any toys you want to keep". I had a hard time with this because we don't have a lot of money and to throw away toys that we spent money on was and is really hard. But I have done this, and it worked on my two oldest, but not the youngest... he claimed he didn't care (until two days later when he threw a fit).
My biggest mistakes along the way have been doing it for them, not rotating toys (kids get tired of the same toys - but if you have three storage boxes of toys you can rotate through this will help keep them interested and motivated to clean up - the hard part of this is you need to keep up with the rotation), and lastly I have tended to "nag" or "referee" them during clean-up time. I no longer do this. We have clean up twice a day (in afternoon and before bed). I set the timer, cheerfully say, "Pick up whatever you played with today and let me know if you beat the timer". And I walk away. I also pitch in and help them a couple times a week because I know it's hard for them to do it ALL the time. I also have a chore chart they can put stickers on and earn more toys back, or a reward (like getting movie rentals, pizza night or dollar store choice).
Best of luck to you. Most of the resources I looked at said when kids start to not care enough to pick up they are either bored with their toys, or have too many and it's just time to thin it out.
Wishing you success and Peace about it
M.