I'll go in detail just for the sake of it.
Okay let's go over each icon one by one.
The First Icon
It's the Check Meter for both versions. HOWEVER the cycles of check meter screens is this:
Ver. 3 -> Hunger/Happiness, Discipline, Name/Age/Weight, Gender/Generation, Gotchi Points, Username
Ver. 4 -> Hunger/Happiness, Discipline, GUTS/Skill Points, Name/Age/Weight, Gender/Generation, Gotchi Points, Username
Also the labels saying "Hungry" and "Happy" are closer to the hearts on version 4. Also, on v4, you can earn up to 99 999G, as opposed to the 9 999G maximum on ver. 3.
The Second Icon
It's the Feeding Time icon for both versions. There are slightly different food choices though, like an apple on v3 but an orange on v4. And yes there are different food items you can buy!
The Third Icon
It's the Toilet icon for both versions. I've noticed though that the toilet is slightly different looking to version 3. On v4, it looks identical to the Entama's icon, in which the seat looks like there's a wedge cut out of it, but on v3 it looks like there's a black hole in the middle of the seat.
It functions in the same way, but I've noticed it does the job quicker on v4 (on my v1-v3 it paused for a second and then flushed the screen)
The Fourth Icon
The Activity icon for both versions, and it has the same options for both versions. I've noticed on v4 that for example you choose "Password" then you get out of the menu, the next time you go on the menu "Password" will be highlighted, as opposed to v3 in which "Game" will always be highlighted upon entry regardless of what you chose last.
The games are, of course different. I'll list them all down.
Ver. 3 -> Get *musical note*, Bump, Flag, Heading, Memory, Sprint
Ver. 4 -> Jumping Rope, Mimic, Shape, Dance, Flag
And the stages at which you unlock the games:
Baby-> Get *musical note* (v3), Jumping Rope (v4)
Child -> Bump (v3), Mimic (v4)
Teenager -> Flag (v3), Heading (v3), Shape (v4), Dance (v4), Flag (v4)
Adult -> Memory (v3), Sprint (v3)
v3 has more button mashing games than v4. And the point awarding, if you beat the games fully as an adult (this is to memory):
Get *musical note*: 300G
Bump: 2400G if thin, 1200G if overweight
Flag: 400G
Heading: 400G
Memory: 400G
Sprint: (unsure, never beaten)
Jumping Rope: 400G
Mimic: 400G
Shape: 400G
Dance: 400G
Flag: 400G
And v4, if you have 3 stars in wealth you will get 500G from beating certain games instead of 400.
The items are of course, different.
The shop codes are different too.
The passwords are from the v4 Tamagotchi Town instead of the v3 Tamagotchi Town.
And the souvenirs are different.
The Fifth Icon
IR Communication on both versions. The options are different though.
Ver. 3 -> Ver. 3, Others
Ver. 4 -> Work, PC, Ver. 4, Others
Version 4 can go to preschool, school and work and uses 14 digit codes to connect to Tamagotchi Town, using the digits 0-9 and letters A-F as well as a 5 letter username using the 26 letters of the English alphabet as well as ! and ?.
Version 3 cannot go to work and uses only a 5 letter username to connect to Tamagotchi Town, using the letters of the alphabet as well as ! and ?.
The Sixth Icon
Discipline on both versions and functions exactly the same.
The Seventh Icon
Medicine on both versions and functions exactly the same.
The Eighth Icon
Now we get interesting! Lights on v3, Mail on v4.
On Ver. 3 you can switch the lights on or off. You'll need to switch the lights off for it when it goes to sleep.
On Ver. 4, you cannot. Instead, you have mail. The star is fortune cookie, the envelope is normal mail and the [ ! ] is important mail. You can receive both good and bad mail and your Tamagotchi is affected by the fortune cookie. The important mail is for sending your Tamagotchi to school, work, etc.
The Ninth Icon
It's the notebook on both versions and functions the same.
However, different presents, as I've mentioned before.
On the Point screen, where you can donate Gotchi Points to the Gotchi King, the numbers are smaller on Ver. 4 and the amounts you are required to donate are different.
Ver. 3 -> 5 000G, 10 000G, 30 000G
Ver. 4 -> 20 000G, 70 000G, 200 000G
When you get the required amount on v3, the screen will flash "Thankyou Very Much" and you are given the 10 digit passcode for Tamagotchi Town (which you have a pen and paper handy to write it down as it comes and goes quickly). You get the souvenir same way as a normal souvenir.
When you get the required amount on v4, the screen will flash "Thankyou Very Much" and then "VISIT!! TAMA TOWN" will appear. Then you need to go and manually get your 14 digit password, like you normally would to enter Tamatown. And as for the souvenir... I won't spoil the surprise!!!
The Tenth Icon
Attention icon on both, and it functions the same way.
Other Differences
The Stereo- "Stereo" on v4, "Boombox" on v3. On v3, using the one CD (with only one track) can blow up your boombox. On v4, using one of 3 CDs (with two tracks each and you can select them) cannot blow up your stereo.
Instruments- More instruments on v4 and your Tamagotchi can even make mistakes while playing them (on v3, they can't)!
Ball- Different times for falling off the ball on each version (v4 can fall off when gets to the side, v3 can fall off when in the center of the screen)
I think I have covered pretty much everything now.