test: bump PHPStan to level 4

phpstan.neon

@@ -3,4 +3,4 @@ parameters:
         - src
 
     # The level 8 is the highest level
-    level: 0
+    level: 4

src/Calendars/YasumiUkCalendar.php

@@ -4,27 +4,30 @@
 
 use Carbon\Carbon;
 use Worksome\UkTaxAllowance\Contracts\UkCalendar;
-use Yasumi\Provider\AbstractProvider;
 use Yasumi\Provider\UnitedKingdom;
+use Yasumi\ProviderInterface;
 use Yasumi\Yasumi;
 
 class YasumiUkCalendar implements UkCalendar
 {
     protected const FUTURE = 1;
     protected const PAST = -1;
 
+    /** {@inheritdoc} */
     public function isWeekendDay(Carbon $date): bool
     {
         return $this->getCalendar($date)
             ->isWeekendDay($date);
     }
 
+    /** {@inheritdoc} */
     public function isHoliday(Carbon $date): bool
     {
         return $this->getCalendar($date)
             ->isHoliday($date);
     }
 
+    /** {@inheritdoc} */
     public function isWorkingDay(Carbon $date): bool
     {
         $ukCalendar = $this->getCalendar($date);
@@ -62,7 +65,7 @@ protected function closestWorkingDay(Carbon $date, int $direction): Carbon
         return $workingDay;
     }
 
-    private function getCalendar(Carbon $carbon): AbstractProvider
+    private function getCalendar(Carbon $carbon): ProviderInterface
     {
         return Yasumi::create(UnitedKingdom::class, $carbon->year);
     }

src/UkTaxAllowanceCalculator.php

@@ -54,6 +54,7 @@ public function monthlyEndDatesBetween(Carbon $start, Carbon $end): Collection
             return collect([$dateEnd->copy()]);
         }
 
+        /** @var Collection<Carbon> $months */
         $months = collect(
             CarbonInterval::day()
                 ->toPeriod($dateStart, $dateEnd)
@@ -71,7 +72,7 @@ public function monthlyEndDatesBetween(Carbon $start, Carbon $end): Collection
 
     public function weekly(Carbon $start, Carbon $end): int
     {
-        return collect($this->weeklyEndDatesBetween($start, $end))->count();
+        return $this->weeklyEndDatesBetween($start, $end)->count();
     }
 
     public function monthly(Carbon $start, Carbon $end): int

src/UkTaxAllowanceServiceProvider.php

@@ -8,7 +8,7 @@
 
 class UkTaxAllowanceServiceProvider extends ServiceProvider
 {
-    public function register()
+    public function register(): void
     {
         $this->app->bind(UkCalendar::class, function () {
             return new YasumiUkCalendar();