diff --git a/NEWS.md b/NEWS.md
index ef905da..05aebb3 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -2,5 +2,5 @@
 
 ## Added
 * Adds `apus` object with the functions `addField`, `trainModel` and `optimizeFertilizerChoice`
-* Adds for the cost function module 1,5 and 6
+* Adds for the cost function module 1,4, 5 and 6
 * Adds default table for `cultivations`, `fertilizers`, `parameters` and `fines`
diff --git a/R/model.R b/R/model.R
index a29128d..5c23a92 100644
--- a/R/model.R
+++ b/R/model.R
@@ -168,6 +168,10 @@ calculateCost <- function(doses, fields, fertilizers, fines, reduce_batches = TR
   module1 <- calculateCostModule1(doses, fields, fertilizers)
 
 
+  # Module 4: Cost of applying fertilizers ----------------------------------
+  module4 <- calculateCostModule4(doses, fields, fertilizers)
+
+
   # Module 5: Revenue from harvested crops ----------------------------------
   module5 <- calculateRevenueModule5(doses, fields, fertilizers)
 
@@ -177,7 +181,7 @@ calculateCost <- function(doses, fields, fertilizers, fines, reduce_batches = TR
 
 
   # Combine the modules -----------------------------------------------------
-  cost <- torch::torch_zeros(dim(module1)) + module1 - module5 + module6
+  cost <- torch::torch_zeros(dim(module1)) + module1 + module4 - module5 + module6
 
 
   # Convert to € / ha -------------------------------------------------------
@@ -215,6 +219,35 @@ calculateCostModule1 <- function(doses, fields, fertilizers) {
   return(module1)
 }
 
+# Module 4: Cost of applying fertilizers  -----------------------------------
+calculateCostModule4 <- function(doses, fields, fertilizers) {
+
+  # Sum dose per fertilizer  per field --------------------------------------
+  fields.b_area <- torch::torch_unsqueeze(fields[,,1], -1)
+  fields.fertilizers.dose <- fields.b_area * doses
+
+
+  # Calculate number of applications per field ------------------------------
+  fertilizers.p_app_capacity <- fertilizers[,,10]
+  fields.fertilizers.p_app_capacity <- torch::torch_unsqueeze(fertilizers.p_app_capacity, 2)
+  fields.fertilizers.p_app_capacity <- torch::torch_repeat_interleave(fields.fertilizers.p_app_capacity, repeats = dim(fields.fertilizers.dose)[2], dim =2)
+  fields.fertilizers.applications <- torch::torch_ceil(fields.fertilizers.dose / fields.fertilizers.p_app_capacity)
+
+
+  # Calculate cost of applications -------------------------------------------
+  fertilizers.p_app_cost <- fertilizers[,,9]
+  fields.fertilizers.p_app_cost <- torch::torch_unsqueeze(fertilizers.p_app_cost, 2)
+  fields.fertilizers.p_app_cost <- torch::torch_repeat_interleave(fields.fertilizers.p_app_cost, repeats = dim(fields.fertilizers.dose)[2], dim =2)
+  fields.fertilizers.cost <- fields.fertilizers.applications * fields.fertilizers.p_app_cost
+
+
+  # Sum cost for farm -------------------------------------------------------
+  fields.cost <- torch::torch_sum(fields.fertilizers.cost, dim = 3L)
+  module4 <- torch::torch_sum(fields.cost, dim = 2L)
+
+  return(module4)
+}
+
 # Module 5: Revenue from harvested crops  ------------------------------------
 calculateRevenueModule5 <- function(doses, fields, fertilizers) {
 
diff --git a/README.md b/README.md
index 379aae3..17c6975 100644
--- a/README.md
+++ b/README.md
@@ -78,7 +78,7 @@ For the v1 version of `apus` we plan to develop to following features:
 * [x] Include cost function for module 1: Purchase of fertilizers
 * [ ] Include cost function for module 2: Disposal of manure
 * [ ] Include cost function for module 3: Cost of storing fertilizers
-* [ ] Include cost function for module 4: Cost of applying fertilizers
+* [x] Include cost function for module 4: Cost of applying fertilizers
 * [x] Include cost function for module 5: Revenue of harvest
 * [x] Include cost function for module 6: Penalties in case of exceeding legal limits
 * [ ] Include cost function for module 7: Cost of greenhouse gas emissions
diff --git a/data-raw/fertilizers.R b/data-raw/fertilizers.R
index 1bbeb3a..0a62fbd 100644
--- a/data-raw/fertilizers.R
+++ b/data-raw/fertilizers.R
@@ -19,6 +19,9 @@ fertilizers[p_type_compost == TRUE, p_p_wcl := 0.25]
 fertilizers[p_name_nl == 'Champost', p_p_wcl := 0.75]
 fertilizers[p_name_nl == 'Rundvee vaste mest', p_p_wcl := 0.75]
 
+fertilizers[, p_app_cost := 100]
+fertilizers[, p_app_capacity := 15000]
+fertilizers[p_type_artificial ==TRUE, p_app_capacity := 1000]
 
 # Export table ------------------------------------------------------------
 usethis::use_data(fertilizers, overwrite = TRUE, version = 3, compress = 'xz')
diff --git a/data/fertilizers.rda b/data/fertilizers.rda
index 5b860b7..c1fd51f 100644
Binary files a/data/fertilizers.rda and b/data/fertilizers.rda differ
diff --git a/man/fertilizers.Rd b/man/fertilizers.Rd
index 938d509..56c8e8f 100644
--- a/man/fertilizers.Rd
+++ b/man/fertilizers.Rd
@@ -5,7 +5,7 @@
 \alias{fertilizers}
 \title{Fertilizers table}
 \format{
-An object of class \code{data.table} (inherits from \code{data.frame}) with 47 rows and 46 columns.
+An object of class \code{data.table} (inherits from \code{data.frame}) with 47 rows and 48 columns.
 }
 \usage{
 fertilizers
diff --git a/tests/testthat/test-002-model.R b/tests/testthat/test-002-model.R
index 4f92774..cb13625 100644
--- a/tests/testthat/test-002-model.R
+++ b/tests/testthat/test-002-model.R
@@ -36,6 +36,14 @@ test_that("Calculate cost for module 1: Purchase of fertilizers", {
   expect_length(as.numeric(module1), farms_count)
 })
 
+
+module4 <- calculateCostModule4(doses, fields, fertilizers)
+
+test_that("Calculate cost for module 4: Cost of applying fertilizers", {
+  expect_contains(class(module4), 'torch_tensor')
+  expect_length(as.numeric(module4), farms_count)
+})
+
 module5 <- calculateRevenueModule5(doses, fields, fertilizers)
 
 test_that("Calculate revenue for module 5: Revenue of harvested crops", {