Maybe look into children's books? They seem to make them fit everything now :)

Just explain that sometimes mummies and daddies can't live happily together. That you and her daddy both love her very much but that your family is happier if you and daddy live apart. After all, that's the truth, right? And if he lives so far away for work then explain that sometimes daddies can't live near their children because they need to work away. Make it all very casual and normal. If you're nervous or sad when you talk about it then she'll pick up on your vibe and think there's something wrong that you're not telling her.

My husband and his first wife divorced when his son was two. When he was five he asked why mom and dad don't live together and we told him that they decided a while ago that it was better for everyone if they lived in different houses. He didn't really have any questions after that. He doesn't remember when they were together. 

So my stepson was 2.5 when my hubby & his ex started divorce proceedings, at 1st they just told him "Mama & Baba aren't going to live together anymore, you will still get to see & live with both of us just at different houses." They had 50/50 custody. He got used to "Mama's house" & "Baba's house", but there was some confusion as to why his mom couldn't stay when she would drop him off. I came on the scene shortly after the divorce & his mother also started dating someone, that 1st year was very confusing for him because my hubs & his ex didn't really have boundaries so they still interacted A LOT, eventually he started asking why they didn't live together, the reason they got divorced was because she realized she was a lesbian after 10 years of marriage, so my husband told his 4 year old that they couldn't still be together because his mom liked girls, then there were conversations about romantic love versus friend/family love, once we got married we talked about people being "a good match", I said sometimes people get together & then realize later that they r not a "good match" & this was what had happened w/ his parents, but his dad & I were a very good match & we wouldn't get a divorce, we would always work very hard to stay a good match. This seemed to make sense to him & we talked about it more once his mom's relationship with her partner ended. Now he lives w/ us 75% of the time & his mom sees him on at least 2 weekends a month & during school vacations, she met someone else & they got married & things r relatively stable over there, he's 6.5 now & doesn't question anything anymore, His dad & I have been together for over 3 years, married for 2, & baby sister is due in 2 weeks! Keeping explanations simple & providing a stable environment r the keys for keeping the confusion & questions at a minimum & it sounds like you're doing that.

Since she's so young, I'd tell her that he moved because of work or something. I know some young kids (depending in how it's explained and their maturity level) feel that it's their fault, and that's obviously something you're trying to avoid.

Just be honest about it but in terms she will understand. My step daughter had the same questions when her mom did this with her step dad. We didnt say anything the first time they separated but this time they are divorcing so we just told her that they arent together anymore and thats why they dont live together anymore. She understands that her mom is with someone else( the guy she cheated with) and that they arent together. Your daughter is young and probably just curious as to why this happened. You dont need to go into to detail why it didnt work out but just say we decided we were better off appart from each other and that's why we dont live together anymore.