for my son's 4 month pics, i had him just in his cloth diaper and laying on his tummy...i did some nakey ones and then put him in a basket. (i mainly just did candid shots,, some in my studio and some in out living room in front of the slider door with natural light. just laid him on his tummy and laid down in front of him and just took pics)
You could get some blocks and spell out FOUR for 4 months.
Lots of different baskets..
Lots of different fabrics/blankets he could lay on for different looks
A beanbag to prop him up in
Let him lay in the grass on his tummy
here's someone's from a 4 month shoot
Amanda Joy Photography - Columbia SC Child Photographer
search on flickr too. GL, HTH!!