d4 · independent software studio · 
England
d4 is the studio of Davinder Singh, building native apps, network tools and sovereign cyber defence, and the infrastructure they run on. Fast, private by design and honest about what they do. No trackers, no cloud middlemen, no fuss.
One design language across everything d4 makes. Three apps so far, and each earns its place.
Your pfSense firewall, on iPhone and iPad.
Monitor, control and analyse a pfSense firewall over SSH, with live dashboards, full rule editing and a Wireshark-style packet analyser. Nothing is installed on the firewall and nothing passes through a cloud service. Your credentials never leave the device.
The diary made for driving instructors.
A private iPhone app for UK driving instructors. Take the lesson fee with Tap to Pay on iPhone before the pupil is out of the car, book lessons around your real availability, track every pupil's progress on the DVSA scale, log mileage and expenses, get your Self Assessment figures totalled, and score lessons hands-free in CarPlay. No accounts, nothing shared, all on your device.
Ultimate stage audio playback.
A no-nonsense backing-track player built for live musicians. Fast, responsive playback with zero distractions, and high-contrast visuals that stay readable under stage lighting. Built for the British Bhangra group B21, and free for any performer.
More is on the bench. New work appears here when it is ready, and not before.
A studio that builds software and, when it makes sense, the infrastructure it runs on.
iOS and iPadOS apps that feel at home on the platform, with credentials on the device and nothing in the cloud. pfAdmin is the first.
Firewalls, routing, VPN and monitoring. The unglamorous plumbing everything else depends on, done properly.
Private AI platforms and enterprise deployments on hardware you control, with your data kept inside your walls.
Least privilege, encryption and host-key pinning, decided at the start and not patched in afterwards.
Beyond the apps, d4 designs and builds platforms and sites for others. A few from the workbench.
Benchmarking security operations maturity and compliance readiness, mapped to NIST CSF, ISO 27001, MITRE ATT&CK, NCSC CAF and more.
A free DEFCON 658 and Defence Cyber Certification self-assessment, scoring you against Def Stan 05-138 the way an assessor would.
The site for the British Bhangra group B21: thirty years of Handsworth sound, designed and built by d4.
Short, plain-English reads on cyber and sovereignty news, with UK security and Secure by Design front of mind. Read them all
Latest post
31 july 2026 · ransomware and cybercrime · vulnerabilities and patching
Amazon has linked four npm supply chain compromises, including debug, chalk and axios, to a North Korea-linked group that phished a single trusted maintainer. Plus: an actively exploited hardcoded credential in Cisco's firewall manager, ShinyHunters' extortion claim against EY, and new NCSC guidance on surviving a disruptive cyber-attack.
Read the full five minutes →More from Decrypted
05 · About
d4 is a studio of one, run by Davinder Singh. The design, the code, the servers and the support inbox all pass through the same hands.
Nothing here is outsourced, tracked or rented out. The studio would rather ship one considered thing than ten careless ones. If it carries the d4 mark, it was built here, and d4 stands behind it.
· Davinder Singh, founder
06 · Contact
A project, an app, or support for one you already use. Every message is read and answered by a person.
Or use the Feedback button in the corner. It lands on the studio desk.
