Task never defined: Moving from gulpV3 to gulpV4

I've had to upgrade an old project from gulp version 3.9.1 to gulp version 4 which required me to rewrite my tasks. I've followed the documentation for moving to gulp.series and moving to name functions but I am now still getting errors on run. Below is my code which I use for watching styles in two different languages.

var fontName = "project-icons",
    gulp = require("gulp"),
    sass = require("gulp-sass"),
    sourcemaps = require("gulp-sourcemaps"),
    iconfont = require("gulp-iconfont"),
    iconfontCss = require("gulp-iconfont-css");

var sassOptions = {
    errLogToConsole: true,
    outputStyle: "expanded"
};

function iconfont(done) {
    gulp.src(["./icons/*.svg"])
        .pipe(
            iconfontCss({
                fontName: fontName,
                path: "sass",
                targetPath: "../sass/static/icons/_icons.sass",
                fontPath: "./fonts/",
                cssClass: "icon"
            })
        )
        .pipe(
            iconfont({
                formats: ["ttf", "eot", "woff", "woff2", "svg"],
                fontName: fontName,
                normalize: true,
                fixedWidth: true,
                centerHorizontally: true
            })
        )
        .on("glyphs", function(glyphs, options) {})
        .pipe(gulp.dest("./fonts/"));
    done();
}

function desktop_styles() {
    return gulp
        .src("./sass/_en/style.sass")
        .pipe(
            sourcemaps.init({
                loadMaps: true,
                identityMap: true,
                sourceRoot: "../css/"
            })
        )
        .pipe(sass(sassOptions).on("error", sass.logError))
        .pipe(sourcemaps.write())
        .pipe(gulp.dest("./css/"));
}

function desktop_styles_rtl() {
    return gulp
        .src("./sass/_ar/style.sass")
        .pipe(
            sourcemaps.init({
                loadMaps: true,
                identityMap: true,
                sourceRoot: "../css/"
            })
        )
        .pipe(sass(sassOptions).on("error", sass.logError))
        .pipe(sourcemaps.write())
        .pipe(gulp.dest("./css/lang/rtl"));
}

function mobile_styles() {
    return gulp
        .src("./sass/en/mobile/style.sass")
        .pipe(sass(sassOptions).on("error", sass.logError))
        .pipe(gulp.dest("./css/m"));
}

function mobile_styles_rtl() {
    return gulp
        .src("./sass/rtl/m/style.sass")
        .pipe(sass(sassOptions).on("error", sass.logError))
        .pipe(gulp.dest("./css/lang/rtl/m"));
}

function watch_all(){
    gulp.series(
        mobile_styles,
        mobile_styles_rtl,
        desktop_styles,
        desktop_styles_rtl,
        function(done) {
            gulp.watch("./sass/**/*.sass", gulp.series(mobile_styles));
            gulp.watch("./sass/**/*.sass", gulp.series(mobile_styles_rtl));
            gulp.watch("./sass/**/*.sass", gulp.series(desktop_styles));
            gulp.watch("./sass/**/*.sass", gulp.series(desktop_styles_rtl));
            done();
        }
    )
};

function watch_AllDesktop(done){
    gulp.series(desktop_styles, desktop_styles_rtl, function() {
        gulp.watch("./sass/**/*.sass", gulp.series(desktop_styles));
        gulp.watch("./sass/**/*.sass", gulp.series(desktop_styles_rtl));
        done();
    });

function watch_desk_desktop_styles_rtl(done){
    gulp.series(desktop_styles_rtl, function() {
        gulp.watch("./sass/**/*.sass", gulp.series(desktop_styles_rtl));
        done();
    });

function watch_desktop_en(done){
    gulp.series(desktop_styles, function() {
        gulp.watch("./sass/**/*.sass", gulp.series(desktop_styles));
    });

function watch_mobile_rtl(done){
    gulp.series(mobile_styles_rtl, function() {
        gulp.watch("./sass/**/*.sass", gulp.series(mobile_styles_rtl));
        done();
    });

function watch_mobile_en(done){
    gulp.series(mobile_styles, function() {
        gulp.watch("./sass/**/*.sass", gulp.series(mobile_styles));
        done();
    });

function watch_AllMobile(done){
    gulp.series(mobile_styles, mobile_styles_rtl, function() {
        gulp.watch("./sass/**/*.sass", gulp.series(mobile_styles));
        gulp.watch("./sass/**/*.sass", gulp.series(mobile_styles_rtl));
        done();
    });

When I run "gulp watch_all" I get Task never defined. Is there something wrong with the functions or my code order?

1. All your functions from watch_AllDesktop to watch_AllMobile need a closing bracket }

2. Your main Problem is, that you don't export your task

Intern tasks no needed to export, but all your tasks, which should be runable per console must have an export.

Put for example following export at the end of the gulpfile.js:

exports.watch_all = watch_all;

After the export you can proof with gulp --tasks which tasks are available for you

---

edit (comment question)

Your watch should have no callback, because the watching should run permanent and not only once.

function watch_all(){
    gulp.watch("./sass/**/*.sass", gulp.series(mobile_styles, mobile_styles_rtl, desktop_styles, desktop_styles_rtl ));
}

For further questions, please open new question with small examples and the affected code. thx