Hey I'm looking for someone if interested, that could help me create a formula for a probability system in my game I play. Might be complex idk. I'm just a gamer trying to get an edge in my game I play. Base item + base item has a 75% chance succeeding to level 1 item. Base item + level 1 item has a 50% chance of succeeding to level 2. Base item + level 2 item has 44% chance succeeding to level 3 item. Base item + level 3 item has 30% chance succeeding to level 4 item. If failing, u have a 40% of downgrading or 60 % chance of staying at current level and u lose the base item. At Base + base, u can't downgrade further, u just lose 1 base item.
Just to clarify, it already seems like you have a "probability system". Are you asking how to optimize these probabilities?
What are you looking for a formula for?  Your explanation makes sense for how the system works but I am not sure what you want us to calculate from that info.
by
One can model this with a transition matrix:

1    0     0     0    0
.3   .42  .28     0    0
0   .44  .336  .224   0
0    0    .5    .3   .2
0    0     0    .75  .25

> Looking to figure out how many base items are needed to get to lvl 4 on average

Now slice off the top row, then slice off the left column so you're left with a 4x4 matrix, call it Q.

Let F = (I-Q)⁻¹

The average number of rounds required to reach L4 when starting with base+base is the sum of the bottom row of F, which is approximately 14.08735

Is that what you wanted, or did you wanna know how many base items you'll lose on average?