"rxjs" observable.throw is not a function - Angular4

The error There are multiple modules with names that only differ in casing. is indicating the wrong import is being targeted with how you are trying to use Observable.

The import should be with a capital "O" like:

import { Observable } from 'rxjs/Observable';

This will import the individual Observable operator, which be used in combination with operators such as catch or throw on created Observables.

import 'rxjs/add/operator/catch';
import 'rxjs/add/observable/throw';

To import the full Observable object you'd import it like this:

import { Observable } from 'rxjs/Rx'

Update:

With newer versions of RxJS (5.5+) operators such as map() and filter() can used as pipeable operators in combination with pipe() rather than chaining. They are imported such as:

import { filter, map, catchError } from 'rxjs/operators';

Keep in mind terms such as throw are reserved/key words in JavaScript so the RxJS throw operator is imported as:

import { _throw } from 'rxjs/observable/throw';

Update:

For newer versions of RxJS (6+), use this:

import { throwError } from 'rxjs';

and throw an error like this:

if (error.status === 404)
    return throwError( new NotFoundError(error) )

In RxJS 6, Observable.throw() is replaced by throwError() which operates very similarly to its predecessor. So, you can replace from Observable.throw(error) to only throwError(error) by importing:

import { throwError } from 'rxjs';

Checkout this link for further reference: https://www.metaltoad.com/blog/angular-6-upgrading-api-calls-rxjs-6