Angular 4 - Cannot find name 'HttpClient'

I'm trying to access a JSON feed but after making the changes from the documentation I get the following error

"Cannot find name 'HttpClient'"

I've looked over the tutorial a few times but I'm struggling to find why I get this error.

My component where I perform the Http request.

rooms.parent.component.ts

import { Component, OnInit } from '@angular/core';

@Component({
 /.../
})

export class RoomParentComponent implements OnInit {
  results: string[];

  // Inject HttpClient into your component or service.
  constructor(private http: HttpClient) {}

  ngOnInit(): void {
    // Make the HTTP request:
    this.http.get(/.../).subscribe(data => {
      this.results = data;
    });
  }
}

app.module.ts

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import {HttpClientModule} from '@angular/common/http';
import { AppComponent } from './app.component';
import { RoomParentComponent } from './rooms.parent.component';

@NgModule({
  declarations: [
    AppComponent,
    RoomParentComponent
  ],
  imports: [
    BrowserModule,
    HttpClientModule,
  ],
  providers: [],
  bootstrap: [AppComponent]
})

export class AppModule { }

How do I resolve the above error and inject the HttpClient into the constructor?

Edit: My Angular version is 4.3.2