Excluding torchflower seeds (requires Sniffers which take 50-60 minutes to hatch and grow), the seeds you need to get for Obtain 4 Unique Seeds are:

• Wheat Seeds

• Melon Seeds

• Pumpkin Seeds

• Beetroot Seeds

• Nether Wart

Nether Wart has been added to this list, but Cocoa Beans still do not count, despite visually looking like a seed. 

Obtained by breaking:

• Short Grass

• Tall Grass

• Fern

• Large Fern

on a 12.5% chance, meaning you need to break ~8 grass per wheat seed. 

Grass-like blocks that do NOT drop wheat seeds are:

• Short Dry Grass

• Tall Dry Grass

• Bush

• Firefly Bush

Using bone meal on moss in grassless seeds can generate Short/Tall grass for wheat seeds. 

You can also obtain wheat seeds by breaking crops generated in villages (most pertinently, desert and snowy plains villages, since these are the ones most likely to generate not near breakable grass). Farm plots are weighted a bit more than larger village features/houses, but the chance a certain farmland is wheat is different depending on the village type:

• Desert – 72%

• Snowy Plains – 18% (NB need to check if it's impossible to get wheat seeds from Snowy Plains villages). 

(90% for Savanna villages, 56% for Taiga, and 50.4% for Plains.)

Chance for farmland to be melon:

• Savanna: 10%

• Desert: 8%

• (All other villages 0%)

Melon stems can also be found in Woodland Mansions.

Melon seeds can be obtained from mob spawner room (Monster Room) chests at a 18.5% chance per chest, and from mineshaft minecart chests at a 27.3% chance. This is exactly the same chance for Pumpkin Seeds and Beetroot Seeds. The chance does not change for minecarts in a sulfur cave. Finding mineshafts with pie chart

Melon seeds can be sold by the Wandering Trader for an emerald at a 7% chance.

Melon blocks

Melon blocks can be broken into Melon Slices which in turn can be crafted into Melon Seeds. Melon blocks can generate in jungle biomes (rarely below y63 and getting less frequent as y increases, this is a similar distribution to pumpkins); and as piles in Savannah villages. 

The exact biomes that melons can generate as vegetation features are:

• Jungle

• Sparse Jungle

• Bamboo Jungle

For Pumpkin stems, the village type needs to be Taiga. There is a 40.6% chance to get pumpkin seeds from a Taiga village chest. 

Chance for village farmland to be pumpkin:

• Taiga: 30%, but pumpkin blocks are nearly guaranteed.

• (All other villages 0%)

See graph for melon seeds, above. As with melons, using bone meal will accelerate the growth 2-5 stages, but will not force the stem to grow a fruit. 

Can be found as stems in Woodland Mansion stem room.

Pumpkin blocks

Can generate as piles in Taiga villages. Shearing a pumpkin might be quicker than crafting if you already have shears. 

Pumpkins can generate as part of vegetation features, but unlike melons are much more common. They can generate in any overworld biome with the EXCEPTION of:

• Frozen Peaks

• Jagged Peaks

• Stony Peaks

• Mangrove Swamp

• Meadow

• Lush Caves

• (Sulfur Caves?)

HOWEVER, pumpkin patches require grass blocks, so stony or sandy biomes also effectively won’t spawn pumpkins. As a general rule, biomes with grass (excluding meadows but including gass covered by snow) can generate pumpkin patches. 

Seeds can be obtained from Monster Room chests and Mineshaft minecart chests at 18.5% and 27.3% respectively (same as Beetroot and Melon chances). 

Pumpkins can be obtained from Shipwreck food chests at a 14.4% chance.

Pumpkins can be found in Pillager Outpost tents (4 pumpkins in tent 2). (Outposts can have 0-4 surrounding structures, tent 2 is 1 of 7 options.)

Wandering Traders have a 7% chance to sell Pumpkins for an emerald.

(While you can find carved pumpkins in Mansions and Outposts, these are useless for the purpose of obtaining seeds. You cannot craft a carved pumpkin into seeds.)

Plains and Desert are the only village types that can have beetroot in their farms:

• Plains: 5.6% 

• Desert: 20%

Breaking planted beetroot in any stage will yield 1 beetroot seed (and if fully grown, 1-4 seeds). 

However, Snowy Village chests can have beetroot seeds at a 66.3% chance. These are the snowy village houses that can have these chests (Small Houses 5&6):

Suspicious gravel found in trail ruins (NOT ocean ruins) has a 2.2% chance to drop a beetroot seed. Can work as a hail mary or if you need to overlap with brushing gravel anyway (e.g. obtain pottery sherd, getting Careful Restoration for 30 advancements etc.). Generally mineshaft is just better. 

Monster Room and Mineshaft chests and minecart chests have an 18.5% and 27.3% chance to have beetroot seeds respectively. You can also get them from Woodland Mansion and End City chests (13.5% and 20.4% respectively). 

Wandering Traders have a 7% chance to sell beetroot seeds for an emerald.

Nether Wart

Nether Wart can be found in 2 places: Housing Bastion centres and Nether Fortresses. 

Housing Bastions are guaranteed to have nether wart in the centre, since all the centre types have at least 1 nether wart there, which makes it a more reliable choice for nether wart. 

Forts have nether wart (1) in the bad part in one of the stairs that can generate. These stairs always go down towards the good part, so if you can see the bad part move diagonally down towards the good part you can dig down in there to get nether wart. (2) As a 17.9% chance in the chests that can generate in corners of the bad part. A Fort with 4 chests has a 53.57% chance of having netherwart in the chests. 

Draftout guarantees nether wart in either of the close nether structures. This means that if your closest bastion isn't Housing, there will be a stair room in the Fort with nether wart.