MongoDB 4 Shards in a Sharding Cluster but there is only using 3 and it's not well distribuited

Question

I have a shard cluster with 4 different shards. I'm testing different configurations and shard keys.

I have a collection with 5,000,000 of documents and I'm sharding the collection. But there are only 3 shards in use, and there are 2 with a lot of chunks and the other one has only 1 chunk.

sh.status()

is like that:

sharding version: {
    "_id" : 1,
    "minCompatibleVersion" : 5,
    "currentVersion" : 6,
    "clusterId" : ObjectId("55807b06780a890b28354352")
}
  shards:
    {  "_id" : "sh1_rs1",  "host" : "sh1_rs1/192.168.0.126:27018,192.168.0.189:27017,192.168.0.190:27018" }
    {  "_id" : "sh2_rs1",  "host" : "sh2_rs1/192.168.0.222:27018,192.168.0.229:27018,192.168.0.59:27018" }
    {  "_id" : "sh3_rs1",  "host" : "sh3_rs1/192.168.0.188:27018" }
    {  "_id" : "sh4_rs1",  "host" : "sh4_rs1/192.168.0.228:27018" }
  balancer:
    Currently enabled:  yes
    Currently running:  no
    Failed balancer rounds in last 5 attempts:  0
    Migration Results for the last 24 hours: 
        8360 : Failed with error 'moveChunk failed to engage TO-shard in the data transfer: migrate already in progress', from sh1_rs1 to sh4_rs1
  databases:
    {  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }
    {  "_id" : "test",  "partitioned" : false,  "primary" : "sh1_rs1" }
    {  "_id" : "infonavit",  "partitioned" : false,  "primary" : "sh1_rs1" }
    {  "_id" : "test_shard",  "partitioned" : true,  "primary" : "sh1_rs1" }
        test_shard.coll
            shard key: { "_id" : 1 }
            chunks:
                sh1_rs1 1
                sh2_rs1 1
                sh3_rs1 1
            { "_id" : { "$minKey" : 1 } } -->> { "_id" : ObjectId("557f394e256393d0ee325b14") } on : sh2_rs1 Timestamp(2, 0) 
            { "_id" : ObjectId("557f394e256393d0ee325b14") } -->> { "_id" : ObjectId("5581969fe3a93c05ea233e56") } on : sh3_rs1 Timestamp(3, 0) 
            { "_id" : ObjectId("5581969fe3a93c05ea233e56") } -->> { "_id" : { "$maxKey" : 1 } } on : sh1_rs1 Timestamp(3, 1) 
    {  "_id" : "infonavit_cartera",  "partitioned" : true,  "primary" : "sh1_rs1" }
        infonavit_cartera.creditos
            shard key: { "saldo_actual" : 1 }
            chunks:
                sh1_rs1 43
                sh2_rs1 1
                sh3_rs1 37
            too many chunks to print, use verbose if you want to force print

Do you have any idea why this is happening?