Hi Sis, I went through this worry also before and I know 20+ hours sounds like a lot so it can feel quite scary. At 4 months, babies can still have some long sleep stretches especially if they had a growth spurt or just recovering from something, but usually we also look at whether they are feeding well and having enough wet diapers. If baby is waking up for feeds, active when awake, and gaining weight normally, then it can still be within normal range for some babies. But if baby feels very hard to wake, not feeding well, or you feel something is off, then I would bring to PD just to check and give you peace of mind.
At 4 months, babies do sleep a lot, but 20+ hours a day is on the higher end and worth paying attention to, especially if she seems hard to rouse for feeds or is not feeding well. It could be nothing, but it might also be worth bringing her to the PD just to rule out anything, since at this age good feeding and weight gain matter quite a bit. Better to get a professional to assess and have peace of mind than to wonder.