Having a Snowflake eel that has grown from 20" to 26" in a year... It is of my opinion that it will be TIGHT QUARTERS for a Snowflake or a Zebra unless you get a juvenile and are using the 90 as temporary quarters.
I have a 200g 6.5ft long tank. Although the eel spends the majority of daylight hours in it's lair, mine goes for a daily swim and I suspect it will swim quite a bit at night. Snowflakes/Zebra's can get up to about 3ft long which will be awfully cramped in a 4ft long tank. Also, realize that whether the eel stays hidden or not the waste from the eel. This eel would account for likely 60% or more of your livestock capacity of your tank. Also, the digging and foraging is going to keep your sandbed disturbed and I can foresee your Nitrates being a problem area. My tank filtration is at 16x/hr with a deep sandbed. I have difficulty keeping my Nitrates below 20ppm with only four other 4.5" fish in the tank.
In short, I wouldn't recommend it. I don't have time to check for you now, but I think it was a 'golden tail' Moray get 2ft MAX which may be more suitable. Forget also if they are fish or crustacean feeders. Worthwhile checking out... I can't remember off the top of my head...