For context, I think four-color design is probably impractical and almost definitely unnecessary. The following cards were developed as sort of a practice in hybrid color design. I've read that a card should make sense as a mono-colored card for hybrid costs, and with four colored hybrids as below they should make sense as four separate gold-colored cards. Below are some of the cards I've made - I haven't made one for each combination, as there were some combinations my imagination failed me. If this portion is moderately successful I might try again and post the other combinations at a later time. If you wish to complete the cycle please feel free! Another note - all the cards below are aiming for the uncommon slot as I can't envision four-color cards being common. Please let me know if any of the below cards are successful in being hybrids. Thanks!
Yaga's Rider 2
Creature - Hag (U)
Flash, Flying
When Yaga's Rider enters the battlefield put target attacking or blocking creature on top of its owner's library and then return a creature card with power equal to or less than that creature from your graveyard to your hand.
2/3
Gladitorial Servitude 1
Enchantment - Aura (U)
Enchant Creature
At the beginning of each upkeep, tap enchanted creature. If you control enchanted creature, you may have it fight another target creature. If you do, sacrifice Gladitorial Servitude.
Trip to Hell 1
Instant (U)
Exile target creature. Return that card to the battlefield under its owner’s control at the beginning of the next end step and its owner may have it deal damage equal to its power to a target of his or her choice.
Recollection Specter 2
Creature - Specter (U)
Flying
Whenever Recollection Specter deals combat damage to a player return a card at random from your graveyard to your hand, then shuffle your graveyard.
3/2
Unstable Guardian 1
Creature – Spirit (U)
Vigilance
Whenever you gain life you may switch Unstable Guardian's power and toughness until end of turn
1/8
Rationality of Divinity 1
Enchantment - Aura (U)
Enchant creature
Enchanted creature gets +1/+1 and has hexproof and lifelink.
Image of a Soldier 1
Instant (U)
Until end of turn, target creature loses all abilities and becomes a soldier in addition to its other creature types with base power and toughness 2/2 and first strike.
Draw a card.
Hegemonize
Sorcery (U)
Destroy target land. Gain 2 life.
Ocean-Floor Collector 3
Creature - Octopus (U)
When Ocean-Floor Collector enters the battlefield return target creature card from your graveyard to your hand
When Ocean-Floor Collector dies return target instant or sorcery card from your graveyard to your hand.
3/3
Yaga's Rider 2
Creature - Hag (U)
Flash, Flying
When Yaga's Rider enters the battlefield put target attacking or blocking creature on top of its owner's library and then return a creature card with power equal to or less than that creature from your graveyard to your hand.
2/3
Gladitorial Servitude 1
Enchantment - Aura (U)
Enchant Creature
At the beginning of each upkeep, tap enchanted creature. If you control enchanted creature, you may have it fight another target creature. If you do, sacrifice Gladitorial Servitude.
Trip to Hell 1
Instant (U)
Exile target creature. Return that card to the battlefield under its owner’s control at the beginning of the next end step and its owner may have it deal damage equal to its power to a target of his or her choice.
Recollection Specter 2
Creature - Specter (U)
Flying
Whenever Recollection Specter deals combat damage to a player return a card at random from your graveyard to your hand, then shuffle your graveyard.
3/2
Unstable Guardian 1
Creature – Spirit (U)
Vigilance
Whenever you gain life you may switch Unstable Guardian's power and toughness until end of turn
1/8
Rationality of Divinity 1
Enchantment - Aura (U)
Enchant creature
Enchanted creature gets +1/+1 and has hexproof and lifelink.
Image of a Soldier 1
Instant (U)
Until end of turn, target creature loses all abilities and becomes a soldier in addition to its other creature types with base power and toughness 2/2 and first strike.
Draw a card.
Hegemonize
Sorcery (U)
Destroy target land. Gain 2 life.
Ocean-Floor Collector 3
Creature - Octopus (U)
When Ocean-Floor Collector enters the battlefield return target creature card from your graveyard to your hand
When Ocean-Floor Collector dies return target instant or sorcery card from your graveyard to your hand.
3/3
The missing combinations are:
(B/G)(R/W)
(U/R)(W/B)
(U/G)(B/R)
(U/B)(W/G)
(U/B)(W/R)
(W/B)(R/G)