If you are talking about one of Brooks .25 caliber barrels, it is strong as anything. If it isn't a Brooks, I don't know anything about it.
4F is the way to go.
You will find that in the little guns that without any wadding nothing spectacular will happen.... It will just sorta go woosh. Toilet paper wads can be your friend here. The .177 caliber nano-naval I made for my son sounds like an inch and a half firecracker. You should be able to accomplish that. It isn't going to shake the house or blow out windows, so don't try to do that... if you get that urge think "bigger" :-)
Just rember that a wad is still a projectile, and at short distances it will still be dangerous.
The wad will also smolder, so be careful about where it goes so you don't light anything on fire.
You will find a smoldering wad on the ground about 10 feet +- in front of the gun.