Brass Push>Connect Metric Tube Fitting, 4.00 mm Tube OD To 1/8-28 Male Universal BSPP/BSPT, Straight

Pressure capacity on these push-connect fittings does vary depending on tube size, so 250 psi is the ceiling across the product range, not a flat guarantee for every size. With 4mm tubing specifically, you'll want to stay within whatever your tubing's own rated pressure is, since the fitting instructions call out not exceeding the tubing's temperature and pressure limits. As long as your line stays under both the fitting's max and your tubing's rated threshold, you're in good shape for standard pneumatic instrumentation work.