The 4,000 troops at Tilbury did not represent England's whole field army. They were an initial force positioned to impede Parma's advance after a landing. Regional trained bands and London formations could then enlarge the defence. The problem was a race between Spanish amphibious concentration and English mobilization rather than a static comparison of 4,000 defenders against a much larger army.
Chapter · 32:54-33:19
Tilbury's 4,000 troops were a blocking force, not England's whole army
The four thousand troops at Tilbury formed an initial obstacle to a landing, while England expected regional trained bands to enlarge the field force.