I am trying to overblow in silverstar. Almost worked in 6blow. Is is possiblie in 4blow overblow in Silverstar harmonica?
It's quite possible that a harp like that may well have 1 or 2 overblows out of the box, if you're not unlucky (probably on hole 6 or 5). However, to get them to come out cleanly (or sometimes to come out at all), you'll need to do some reed gapping and other setup tweaks, which aren't tasks to be taken lightly, especially by someone who isn't already and experienced overblow player.
Most experienced players would not recommend anyone who cannot already reliably produce overblows to go playing around with harp setups because 1. you can ruin your harp and 2. you don't really know what you're looking for so you won't be able to identify success.
(My qualifier to this would be, if you have a "junk" harmonica that you don't mind risking breaking, there's no harm in having some fun. You very likely won't improve the harmonica but you might well enjoy yourself, and learn something.)
My advice to you regarding overblows would be this: before you can know if your harp is overblowing, you need to be able to overblow. This may seem like a catch-22, but to really get the feeling for what an overblow feels like, you can "force" even a harmonica that won't usually overblow to do it. And make it a whole lot easier even for a harp that overblows well.
When you overblow, the blow reed "chokes" and the draw reed sounds. To get the harmonica to do this more easily, you can cover the blow reed yourself. There are harmonicas designed specifically with this in mind (for example the suzuki overdrive with holes for the individual chambers), but that's a lot of money for essentially a teaching tool. Instead, take the cover plates off so the reeds are exposed. Then you can just use your fingers to cover reed slots. Be gentle (with your hands and your airflow) and you won't do any damage.
You should notice a few things on holes 1-6
covering the draw reed whilst blowing, or covering the blow reed while drawing gives you a warmer, fuller sound. This is what a valved harp feels like! You'll also be able to bend the blow reeds slighty, with 1-reed valved bends. It's not the same as a full bend, but it's a lot of fun!
if you cover the blow reed and try to blow, the usual note won't sound. But, with some practice, you will be able to overblow, and a lot more easily than usually at that! Hold the note you want in your head, and try and sort of "bend" up to it. I recommend starting on hole 6 (or hole 5 sometimes). you'll be shooting for that minor third up. There are a lot of tutorials out there and opinions about how to achieve that first overblow, and how it "feels". I'll leave that up to you to find what works, but the key is, you'll be able to reach it a lot easier by artificially choking the blow reed.
The advantage real advantage of this is that you can ONLY overblow while the blow is choked, so you can move your mouth around until you manage to make a sound, and that sound will be an overblow. Then you can remember what that feels like and try and make it without the blow reed covered.
Once you can successfully overblow, you'll be able to tell me which holes on your stock silverstar can overblow! And maybe even look into setting it up so they overblow flawlessly.