Add testnet4 frontend support

frontend/src/app/components/amount/amount.component.html

@@ -43,5 +43,6 @@
   <ng-template [ngIf]="network === 'liquid' && !forceBtc">L-</ng-template>
   <ng-template [ngIf]="network === 'liquidtestnet'">tL-</ng-template>
   <ng-template [ngIf]="network === 'testnet'">t</ng-template>
+  <ng-template [ngIf]="network === 'testnet4'">t</ng-template>
   <ng-template [ngIf]="network === 'signet'">s</ng-template>
 </ng-template>