Hi!
Nice to see entrepeneurs giving love to the b4x products.
Well so.... ERPs are database centric, you should choose the one that fits you best. Do not go for SQLite, is nice for small programs but an ERP going for 20 + users will require a full fledged db, for example: MySQL, MariaDB, SQL Server (my personal favorite), Postgres SQL being this last one used for many big companies.
Then. With b4j you have 2 options: ui and non ui. Non ui is for webpages and ui is for desktop apps. The later is faster to code but you will have to create apps for Android and ios, if you use webapps i will be much slower to develop but you code once and run everywhere.
Look at Erel videos on how to code, they are very good for beginners