Best Solution for Using old PBX

We have a client interested in converting to VoIP but they would like to keep their current PBX (non IP) solution they have in place.

What’s the best VoIP hardware to handle this? They currently have 4 pots lines coming into their current PBX.

What are they actually hoping to gain from using VoIP?

If their POTS lines are doing what they need, then VoIP external to the PBX is irrelevant.
If their current switch is digital, VoIP is not going to offer any benefit internally. Being honest, adding four analogue lines into an Asterisk system will cost rather less than paying a year's maintenance on the existing PBX just to support the lines.

I'm guessing that their internal system is not overly large (<50 extensions?). At that size the cost of linking an existing PBX to an IPPBX using analogue ports is going to be more expensive than using the same ports to connect 30 existing analogue desk phones if only because of the complexity.

Yeah, need to know what they want VOIP to do for them - cheaper calls ? teleworkers ? site-to-site connectivity ?

How old is their system, what is it ? how many users - what they need it to do ?