map.options.minZoom = 4;
map.options.maxZoom = 17;

var topLayer = L.tileLayer('https://b.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: 'Source: Esri, i-cubed, USDA, USGS, AEX, GeoEye, Getmapping, ',
    minZoom: 12,
    maxZoom: 17
}).addTo(map);

// Initialize Windy API
windyInit( options, windyAPI => {
    // windyAPI is ready, and contains 'map', 'store',
    // 'picker' and other useful stuff
    // const { map } = windyAPI
    var { map } = windyAPI
    // .map is instance of Leaflet map

the code added to get it working is:

    map.options.minZoom = 4;
    map.options.maxZoom = 17;

    var topLayer = L.tileLayer('http://b.tile.openstreetmap.org/{z}/{x}/{y}.png', {
        attribution: 'Source: Esri, i-cubed, USDA, USGS, AEX, GeoEye, Getmapping, ',
        minZoom: 12,
        maxZoom: 17
    }).addTo(map);
    topLayer.setOpacity('0');

    map.on('zoomend', function() {
        if (map.getZoom() >= 12) {
            topLayer.setOpacity('1');
        } else {
            topLayer.setOpacity('0');
        }
    });
    map.setZoom(14);

and it now works for me. (Adding the minZoom and maxZoom settings to the const options = { ... } settings before the WindyInit, or any code before var { map } = windyAPI in the WindyInit, did not work for me.) Thanks!

(BTW, I'll have a couple more questions later on a couple of other Windy API4 / Leaflet minor issues I'm having that are unrelated to the zoom levels.)

Thank you very very much!!!
justin

map.options.minZoom = 4;
map.options.maxZoom = 18;

Then you can add another tile layer, hidden by default if you start at a higher zoom level:

topLayer =L.tileLayer('https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/{z}/{y}/{x}').addTo(map); 
topLayer.setOpacity('0');       

Create a document ready with and add a zoomend listener that detects if the zoom is 12 or higher. If so, show the 'other' tile layer.

map.on('zoomend', function() { 
    if (map.getZoom() >= 12) {
        topLayer.setOpacity('1');
    } else {
        topLayer.setOpacity('0');   
    }
});

You can now zoom in to a deep level, in this example 18, with a tile layer of your own choice.
Note: this is leaflet related, not specifically Windy API related.

and other people have also asked about this problem in other threads, but no one yet has mentioned a fix (or API update) for this issue so far I believe...