The Osprey is a very nice ship, very affordable and also gets 20% bonus to yield per skill level
Your First Cruiser - So, you finally got Caldari Frigate 4 and Mining 4. The next step of the chain is to get a cruiser. Lucky for you, the Caldari also have the best Aion Gold mining cruiser, the Osprey.
The Osprey is a very nice ship, very affordable and also gets 20% bonus to yield per skill level. I suggest training Caldari Cruiser 3 immediately.
Caldari Cruiser 4 will not be wasted time, but if you just started, you might want to invest the 4 or 5 days it would take you into other skills, like Engineering and Electronics for example.
As for fitting the Osprey, it is pretty straightforward. 3x T2 Miners in the high slots, you can fit a tank in the med slots if you want as the Osprey can handle itself in 0.6 systems with a heavy launcher and some drones.
As usual in the low slots fit as many mining upgrades as you can. One will fit for sure; two will fit with good skills (Electronic 5, Mining Upgrades 4).
Next step is to get Astrogeology 4. It will give you another 20% bonus to your yield and is required for the next step of the chain, which is a Mining Barge (covered later).