Well I played only once with the Romani, and quitted almost immediately then: I do not find that faction in any way particularly appealing, but most of all I find it too easy. (Same reason why I once played, but almost immediately quitted a Qartadastim campaign.)

However there is a rule of thumb:
1) Do you need/have a client ruler there?
2) Do you need the local military potential, really?
3) Are you faced with continuous civic issues, such as squalor/poor economy/low public order/high unrest ?

1) Suggests to keep a type IV.
2) Suggest the same, but if you've fully upgraded the local military structures, your empire is better off with type II here. Unless you have a client ruler there.
3) Suggests to move for the lowest type possible: i.e. type II or III if homeland is unavailable.

Also keep in mind that the Marian reforms are the result of socio-economic developments which can only be represented by building certain buildings (Latifundia mostly) which in turn require type II.

But an experienced Romani player (one who actually has played beyond ~8 turns) can probably give you better advise.