I have a 4 month old should he be able to sit up by now and when should he be able to crawl?

Not so fast!: Most babies will sit up like little "buddhas" between six and nine months. Like all behaviors, there is lots of variability. Some babies bypass crawling altogether - and just start walking! i look for "cruising" - early walking by holding onto furniture around one year - and some form of crawling before then. Good luck with this little human. Lgromkomd.

Developmental mile-: Stones can be found in the parent section of www.Aap.Org & at firstsigns.Org. Gross motor development proceeds head-to-toe the first year. Neurotypical infants get their head & chest up when prone ~4 mos., maintain a "tripod sit, " leaning forward, propping themselves on their arms ~5 mos, maintain a "sit" ~ 6 mos. & get to a sit ~8 mos. There is a range of ages in which 10-90% attain milestones.