MongoDB limit group by results

You can try,

• $match pname condition
• $sort by pname ascending order (optional)
• $group by vname and push root object in items and make array
• $project to show required fields and get 4 objects using $slice

db.collection.aggregate([
  { $match: { pname: "xy" } },
  { $sort: { pname: 1 } },
  {
    $group: {
      _id: "$vname",
      items: { $push: "$$ROOT" }
    }
  },
  {
    $project: {
      _id: 0,
      vname: "$_id",
      items: { $slice: ["$items", 4] }
    }
  }
])

Playground

If you want all objects in root then you can add below pipelines after above pipelines,

• $unwind deconstruct items array to object
• $replaceRoot to replace items object in root

  { $unwind: "$items" },
  { $replaceRoot: { newRoot: "$items" } }

Playground