By farming the 4* units you will have plenty to feed to your 5* units you will be working on. This game will constantly be having events where you can farm 3* or 4* units offered to farm. Once you have a certain 3* unit maxed then you can proceed to feed those units to your 3* or your 5* depending on what you feel like doing first.

What I would suggest is fusing 2* unit cards to your 3* units since 3* card will only require four 2* cards to break a level cap or two 2* with the unit bonus for an even easier time. So if you max a 3* red type Ichigo and get 25 Orbs then you won't be able to get another 25 orbs by maxing him again so if you happen to get more after maxing him, feed that card to either 4* Ichigo or 5* Ichigo for unit bonus. You can only get orbs ONCE per card but they do have different types for same characters of the same rank (i.e. Units who are fully maxed will have a medal on the top right corner of their card and those who aren't maxed won't have anything. When you fully max a unit you'll receive free spirit orbs as a reward. Now here's the reason why you should do that. My main suggestion is to feed as many 4* units as you can AFTER you have fully ascended the 4* or the 3* unit before feeding them to a 5*. So with Global's release of the game I've been saying many questions regarding how you should ascend your 5* units and what people should do with lower rank units.

Bleach brave souls how to awaken