http://www.youtube.com/watch?v=j9t3b_Oxv8gMy only criticism of all then X5 / V6 / V4 chairs is that they are very long, and very wide across front casters. And with the fat tyres are a bit too wide across the tyres for indoor/van use. And you sit high up too.
You own a X5 which is the same as the V6 basically. So you know that its like driving a bus indoors, as its both higher, wider, and longer than your BM2 copy chair? Well I presume you do!
But the V4 is a chair I may look at seriously is I didn't have the ability to build things like my latest lithium powered chair. Its is a contender in any purchase.
Having not seen one in the flesh its hard to know. But I saw and tried X5 (same as V6) and I think it may be wider still... Its hard to find out.