Tuning cello in 4ths

I strongly recommend renting if you can't afford even a beater bass. Even if you do tune a cello in 4ths, the resonances will be different. And more important, the bow hand positions and pressure/speed behavior when playing on bass strings, on a bass, are radically different from a cello.

It's not like switching from a half-size to full-size cello, where only the fingering spacings change. As the comments and answers said, the finger patterns will be quite different as well.

Historically there have been some jazz bassists who have also played cello and tuned it in 4ths, I recall Oscar Pettiford was one of them. He even recorded it a few times.

As @Old_Brixtonian mentioned the scale length is VERY different. The scale length of a 4/4 cello (27.375”) is shorter than even that of a short scale electric bass (30”) and a 3/4 upright is around 42”. If you have some low tension strings for your low register you can probably get away with tuning it an octave above the bass or just do as you see fit.

As for fingering, keep in mind that on the bass we only use 1, 2 and 4 on the left hand and shift more due to the distance between the notes so even if it feels cramped I would suggest you get used to doing that. It’s not going to feel anything like playing a bass but if you simply want to get used to playing an instrument tuned in 4ths it should suffice. Good luck!