diff --git a/src/exo4/exo4.test.ts b/src/exo4/exo4.test.ts
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+++ b/src/exo4/exo4.test.ts
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+import { Country, exclamation, greet, excitedlyGreet } from './exo4';
+
+describe('exo4', () => {
+  describe('greet', () => {
+    it('should greet Alice in french', () => {
+      const result = greet('Alice')(Country.France);
+
+      expect(result).toStrictEqual('Bonjour, Alice');
+    });
+
+    it('should greet Bernardo in spanish', () => {
+      const result = greet('Bernardo')(Country.Spain);
+
+      expect(result).toStrictEqual('Buenos dìas, Bernardo');
+    });
+
+    it('should greet Crystal in english', () => {
+      const result = greet('Crystal')(Country.USA);
+
+      expect(result).toStrictEqual('Hello, Crystal');
+    });
+  });
+
+  describe('exclamation', () => {
+    it('should add exclamation in french style (with a space before "!")', () => {
+      const result = exclamation('Youpi')(Country.France);
+
+      expect(result).toStrictEqual('Youpi !');
+    });
+
+    it('should add exclamation in spanish style (between "¡" and "!")', () => {
+      const result = exclamation('Olé')(Country.Spain);
+
+      expect(result).toStrictEqual('¡Olé!');
+    });
+
+    it('should add exclamation in english style (with no space before "!")', () => {
+      const result = exclamation('Yeah')(Country.USA);
+
+      expect(result).toStrictEqual('Yeah!');
+    });
+  });
+
+  describe('excitedlyGreet', () => {
+    it('should excitedly greet Alice in french', () => {
+      const result = excitedlyGreet('Alice')(Country.France);
+
+      expect(result).toStrictEqual('Bonjour, Alice !');
+    });
+
+    it('should excitedly greet Bernardo in spanish', () => {
+      const result = excitedlyGreet('Bernardo')(Country.Spain);
+
+      expect(result).toStrictEqual('¡Buenos dìas, Bernardo!');
+    });
+
+    it('should excitedly greet Crystal in english', () => {
+      const result = excitedlyGreet('Crystal')(Country.USA);
+
+      expect(result).toStrictEqual('Hello, Crystal!');
+    });
+  });
+});
diff --git a/src/exo4/exo4.ts b/src/exo4/exo4.ts
new file mode 100644
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+++ b/src/exo4/exo4.ts
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+// `fp-ts` training Exercice 4
+// Dependency injection with `Reader`
+
+import { Reader } from 'fp-ts/Reader';
+
+import { unimplemented } from '../utils';
+
+// Sometimes, a function can have a huge amount of dependencies (services,
+// repositories, ...) and it is often impractical (not to say truly annoying)
+// to thread those values through multiple levels of the call stack.
+//
+// That's precisely what dependency injection is meant to solve, and it's not a
+// concept exclusive to OOP!
+// In the world of FP, the so-called `Reader` construct (just a fancy word for
+// a partially applied function) offers precisely those capabilities.
+
+///////////////////////////////////////////////////////////////////////////////
+//                                 SETUP                                     //
+///////////////////////////////////////////////////////////////////////////////
+
+// Let's consider a small range of countries (here, France, Spain and the USA):
+
+export enum Country {
+  France = 'France',
+  Spain = 'Spain',
+  USA = 'USA',
+}
+
+///////////////////////////////////////////////////////////////////////////////
+//                                  ASK                                      //
+///////////////////////////////////////////////////////////////////////////////
+
+// These countries have different rules on how to add exclamation to a sentence:
+// - french speakers will add an exclamation point at the end preceded by a
+//   space: "Youpi !"
+// - spanish speakers begin the sentence with an inverted exclamation point and
+//   finish it by a regular one: `¡Olé!`
+// - english speakers will have no space before the exclamation point at the end
+//   of the sentence: `Yeah!`
+//
+// The following function should take a sentence as input and return a `Reader`
+// that will at some point expect a `Country` and return the sentence with the
+// proper exclamation style.
+//
+// HINT: Take a look at `reader.ask` to access the environment value
+
+export const exclamation: (sentence: string) => Reader<Country, string> =
+  unimplemented();
+
+// Obviously, different countries often mean different languages and so
+// different words for saying "Hello":
+
+export const sayHello = (country: Country): string => {
+  switch (country) {
+    case Country.France:
+      return 'Bonjour';
+    case Country.Spain:
+      return 'Buenos dìas';
+    case Country.USA:
+      return 'Hello';
+  }
+};
+
+// Using the `sayHello` function above, write the `greet` function that
+// delivers a personalized greeting to a person based on their name.
+// The output should look something like `${hello}, ${name}`.
+//
+// HINT: Remember that a `Reader<R, T>` is just an alias for `(r: R) => T`
+//
+// HINT: You can look into `reader.map` to modify the output of a `Reader`
+// action.
+
+export const greet: (name: string) => Reader<Country, string> = unimplemented();
+
+// Finally, we are going to compose multiple `Reader`s together.
+//
+// Sometimes, a simple greeting is not enough, we want to communicate our
+// excitement to see the person.
+// Luckily, we already know how to greet them normally (`greet`) and how to
+// add excitement to a sentence (`exclamation`).
+//
+// Compose those two to complete the `excitedlyGreet` function below:
+//
+// HINT: As with other wrapper types in `fp-ts`, `reader` offers a way of
+// composing effects with `reader.chain`.
+
+export const excitedlyGreet: (name: string) => Reader<Country, string> =
+  unimplemented();
diff --git a/src/exo4/index.ts b/src/exo4/index.ts
new file mode 100644
index 0000000..52d8c4e
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+++ b/src/exo4/index.ts
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+export * from './exo4';