A Handful of New Stuff

Bright Sky Knight
Warrior-Type/LIGHT/Level 3
ATK 800 DEF 400
Once per turn, you can target 1 other level 3 or lower monster you control: During this turn, that monster can attack directly. When this card is sent to the Graveyard: You can target 1 level 3 or lower monster in your Graveyard, except "Bright Sky Knight", then discard 1 card: Special Summon that target in Defense Position.

Wandering Paladin
Warrior-Type/LIGHT/Level 3
ATK 1000 DEF 500
Level 3 and lower monsters you control cannot be targeted by your opponent's card effects. When this card is destroyed by battle and sent to the Graveyard: You can excavate the top 5 cards of your Deck; add 1 level 3 or lower monster from among them to your hand, then shuffle the rest into your Deck.

Radiance Swordmaster
Warrior-Type/LIGHT/Level 4
ATK 1400 DEF 900
If the only monsters you control are level 3 or lower: You can Special Summon this card (from your hand). Level 3 and lower monsters you control gain 500 ATK.

Nightmare Horseman
Fiend-Type/DARK/Level 4
ATK 1500 DEF 0
When this card is Normal Summoned: You can discard 1 Fiend-Type monster from your hand; draw 1 card. When this card attacks a monster: It gains 300 ATK until the end of the damage step.

Soul Stealer
Fiend-Type/FLIP/DARK/Level 1
ATK 0 DEF 0
FLIP: Target 1 monster your opponent controls: It loses 600 ATK and DEF. If the target's ATK or DEF is reduced to 0 by this effect: Destroy it.

Frost Mage
Spellcaster-Type/WATER/Level 4
ATK 1400 DEF 1000
Once per turn, when a Spell Card you control is activated and resolved successfully: You can target 1 monster your opponent controls; it cannot declare an attack, change it's battle position, or be tributed until the end of your opponent's next turn.

Dark Mage Imperion
Spellcaster-Type/DARK/Level 4
ATK 1300 DEF 1000
Once per turn, when you activate a Spell Card: This card gains 300 ATK.

Young Shadow Priest
Spellcaster-Type/DARK/Level 6
ATK 1900 DEF 1500
You can Tribute Summon this card in addition to your Normal Summon/Set by tributing 1 Spellcaster-Type monster, except "Young Shadow Priest" if you activated 2 or more Spell Cards this turn. When this card is Tribute Summoned: You can target 1 Spell Card in your Graveyard; shuffle it into your Deck.

Most seem good.

If 1500 ATK is the typical maximum for Level 4s, then Nightmare Horseman has too much base ATK.

Young Shadow Priest deserves a buff, effect wise... 3 card requirement is hard enough.

The end.