I can't play 3 notes down + 1 note up, repeatedly
Hi, I really have a hard time playing sections that include continuous repetitions of 3 legato notes played in a down bow, then one note up bow, and repeat that several times. All notes are equal, so I must manage to recover in one up bow all the 3 notes I've played down. What tends to happen is that many times I can't recover the full bow and end up running out of bow over time, hahahaha. Besides, the legato notes sound a little ugly and weak.
This applies for any combination really, like 5 eighths up bow and recover in 1 eighth down bow, or viceversa. Nevertheless, if I play them with separated bow strokes, I play all of them very well, fast and precise. Of course, I can't "get away" doing that and I want to do it properly.
These passages are normally fast, slowly I can play them without much trouble.
Most likely you are using too much bow on the down-bows. A useful way to practice this sort of stroke is with a fractional sized bow, if you have one handy (they are very cheap). I have my children's old fiberglass 1/4 sized bow which I pull out for students who struggle with controlling the amount of bow they're using, or who refuse to play in the lower half.
You may also need to lighten the weight of your arm more for that single stroke (or conversely, increase the weight for those 3 notes in accord with what Mary Ellen advised). Because you are moving the bow 3x faster, you can't play with the same pressure as for those previous 3 notes, or you are going to have a huge accent on that one note, and it will be harder to move fast enough. Practice on open strings just making fast whole bows at a lighter pressure to get that feeling, and practice slower bows with more pressure, and then try adding the two together in different up/down combinations.
You are fighting gravity with that one. The down-bow notes are slow and sticky. The up-bow is fast and light, as if some imaginary puppeteer tried to pull it up out of your hand.