I'm trying to improve my bass sight-reading and walking jazz line skills.

My question is, is it best to practice playing finger-style bass lines in 4, is it generally best to play the down beats with just the index finger, or using i-m alternation?

My tendency is to use just the index finger, so that when I anticipate a down beat, or play a triplet riff, I always end up playing index on the downbeat again. Mostly, since I'm going to spend a lot of time practicing exercises, reading notation, etc., I do not want to develop questionable habits. I'm not adverse to practicing multiple ways, if that is what people suggest.

I think the ideal is to always alternate, but I don't.

for 1/4th notes, I use one finger at slow tempos and two fingers at faster tempos (150 bpm+)

The main thing is to play the music as written. The notes in the air don't care which finger you used to pluck the string.

A lot of great bass players only used one finger or only used their thumb.

Try playing the lines both ways and stick with whichever one sounds better. I tend to strictly alternate on every note, but there are always exceptions. Some bass lines it works better to play i-i-m, or i-m-m, etc.

I almost always alternate. Playing with one finger repeatedly just feels weird to me.