Worth checking what the thread is: Suntour Cyclone and VX is 'metric fine' which is M4 x 0.5 mm pitch (metric standard is M4 x 0.7mm pitch).
Metric fine:
https://www.fastenerdata.co.uk/fine-thread
Metric standard:
https://www.fastenerdata.co.uk/metric-thread
I found some cap bolts that fit my Suntour Cyclone:
https://www.ebay.co.uk/itm/384323051054