Babies at 4-5 months are able to push up on their arms and hold their heads up very well, you don't really need anything special to support them. If you want to raise them up so you don't have to get on your belly, you can use your dining room table, or any solid surface. When shooting babies, it is helpful to know the typical stages of development because a 4 month old is vastly different than a 5 month old in terms of what they are likely to do.
For example, a 4 month old can be propped up on her elbows and will hold her head up solidly and you will likely get a smile out of her. (I NEVER wait for the smile, I get something in the "can" first and then work for the smile.) Drape a blanket over her head and have her peek out from under it, classic winner shot. A 5 month old will do the same thing, but will start to roll over because that is their new trick, so you need to have someone watching for that and keep re-positioning them.
A 5 month old also loves their toes and if you put them on their back in a bean bag or large dog bed or some other soft thing, and bring their toes to their hands, you can usually get some cute profile shots of them either holding their feet or sucking their toes. A 4 month old won't do that, a 5 month old will. 4 weeks makes a huge difference.
I wouldn't worry too much about special supporting devices for babies at this age, a boppy pillow if you want, since you will be doing tummy shots. Just don't tire them out, you need to work fast and vary tummy/back shots if they are not happy on their tummy. (I've had mom's say, "She hates tummy time" so I keep it fast and fun and am sure to not over do it. Make sure you have mom nearby to wipe the drool. It is easier to have her wipe it than to worry about it in post, as it is reflective/shiny and a pain. Have fun! Babies are so cute at this age!
