Is it normal that my baby isn't walking yet?

Yes. What is important is how the baby is developing overall. Has the baby reached all other milestones expected prior to walking? Milestones pretty much occur in a certain order, you must be able to roll brfore you sit and crawl before you walk.
Yes. It can be perfectly normal not to learn to walk at all in the first year or even first 15 months or more. For some groups, the average for learning to walk isn't until 13 months. The calendar isn't a concern here. Other things might be. If someone is concerned about developmental delay of any kind, it's wise to discuss this with a pediatrician who has examined the child.
No. Most babies start to cruise around 10-12 months and walk around 10-15 months. If your baby isn't meeting these miletones then you need to discuss this with your pediatrician.
Yes. A 4-month baby is too young to sit without help. Babies usually start walking by 11-13 months. At each regular check-up, the doctor will look at the motor skills gained by baby, to be sure the baby is learning to sit, crawl, stand, and eventually walk. Parents can ask the doctor questions, if they are worried about any motor skills.
Yes. The average age for walking is 9-15 months. If your child is not walking by age 18 months, that is different enough that your doctor will recommend having an evaluation to see if there is a cause for this delay and consider beginning physical therapy to help your child reach this milestone.